You walk into the room, glance up at the indoor unit, and notice it. A steady drip of water running down the wall, or a puddle forming on the floor directly below the air conditioner. It’s one of the most common problems Australian homeowners deal with, and it’s also one of the most misunderstood.
Here’s the first thing to know: water leaking from an indoor air conditioning unit almost always traces back to one of a few root causes, each of which disrupts the normal path that condensate takes from the evaporator coil to the outside drain. The second thing to know: a leaking air conditioner is not something to ignore or run through. A leaking air conditioner won’t get better on its own. The longer it’s left, the more likely you are to be dealing with water damage, mould, or a complete system breakdown, all of which cost far more to fix than the original leak.
This guide walks through every common cause, what you can do yourself, what needs a professional, and how to make sure it doesn’t happen again.
Why Does an Air Conditioner Produce Water at All?
Understanding why your aircon is leaking starts with understanding why it produces water in the first place.
Your reverse cycle air conditioner doesn’t just cool the air, it removes moisture from it. As warm, humid air passes over the cold evaporator coil inside your indoor unit, condensation forms on the coil’s surface. This condensation drips into a tray, which channels the water through a condensate drain pipe that carries it safely outside your home. On a humid day, a single split system can produce several litres of water per hour. That’s normal.
The drain pan sits beneath the evaporator coil and collects the water formed by condensation. From there, it flows into a small pipe called the drain line. If anything disrupts this flow, water backs up and starts to leak.
So the system is designed to produce water and deal with it quietly. When it leaks, something in that drainage process has broken down. The question is what, and that’s what the rest of this guide covers.
One important distinction before we get into the causes: a small amount of water dripping from the outdoor unit’s drain outlet while the system is running in cooling mode is completely normal. Some external water leakage from the outside unit is caused by condensation forming and dripping, which is normal, especially in humid conditions, with no action required. However, constant or excessive leaking may indicate a blocked pipe or refrigerant issue, which requires a professional to attend to. The concern in this guide is water appearing inside your home, from the indoor unit.
Cause 1: Blocked Condensate Drain Line
A blocked condensate drain line is the single most common cause of indoor water leaks in Australian split systems. The warm, humid conditions that make air conditioning so necessary here also create the perfect environment for mould and algae growth inside the drain line. The clearest visual sign is water dripping or streaming from the bottom of the indoor unit, often accompanied by a musty smell near the unit.
Over time, algae, mould, dust, and debris accumulate inside the condensate drain line, eventually blocking it entirely. When the water has nowhere to go, it backs up into the drip tray and overflows into your ceiling, walls, or floor. A blocked drain line can cause significant water damage very quickly, particularly in ducted systems where the leak isn’t always visible until it’s too late.
Signs this is your problem:
- Steady dripping from the bottom edge of the indoor unit
- Musty or mouldy smell when the system is running
- Water staining on the wall below or around the indoor unit
- A gurgling or bubbling sound from the unit, or water stains around the indoor head.
What you can do:
Locate where the drain line exits the building, typically a small PVC pipe coming through the wall near the outdoor unit. When the system is running in cooling mode, you should see water flowing steadily from this pipe. If nothing is coming out, the line is likely blocked.
You can attempt a flush by slowly pouring a mixture of warm water and white vinegar down the accessible end of the drain near the indoor unit, if your system has a drain access point. The mild acidity helps break down algae and mould buildup. A wet/dry vacuum briefly held against the exterior drain outlet can also draw out a partial blockage.
If this doesn’t resolve it, or if you can’t access the drain, a licensed technician can clear it properly with professional equipment and ensure no secondary damage has occurred.
Cause 2: Dirty or Clogged Air Filter
The air filter in your indoor unit traps dust, pet hair and airborne particles before they reach the evaporator coil. When the filter becomes heavily clogged, it restricts the airflow across the coil. With insufficient warm air passing over it, the coil temperature drops below freezing and ice begins to form on its surface. The problem becomes visible when the unit cycles off. The ice melts rapidly and produces far more water than the drain pan can handle in a short period, causing it to overflow.
This is the most preventable cause on this list. A filter that gets cleaned every four to six weeks during heavy use will essentially never cause this problem. A filter left for six months without attention almost certainly will.
What to do:
Switch the system off at the wall, open the front panel, remove the filter, and check its condition. If you can’t clearly see light through the mesh, it needs cleaning. Wash it under warm running water, let it dry completely in a shaded spot, and reinstall it.
If the coil has already frozen, turn the system off and allow it to thaw fully before cleaning the filter and restarting. Attempting to run the system with a frozen coil puts unnecessary strain on the compressor. Air filters should be cleaned every four to six weeks during heavy-use periods.
Our detailed walkthrough on how to clean your air conditioner covers the full process including filter removal, washing, and drying, as well as how to safely clean the indoor casing and check the drain at the same time.
Cause 3: Frozen Evaporator Coil
A frozen evaporator coil is closely related to both dirty filters and low refrigerant, but it’s worth treating as its own cause because it has specific symptoms and a specific first response.
Coils usually freeze due to dirty air filters, blocked condensate lines or drains, broken fans, or refrigerant leaks. If any of these occur, it sets off a chain of events. First, condensation may build up on the evaporator coils. Then, it freezes and the coils are unable to absorb heat.
The leak typically appears when the system cycles off. The ice accumulated on the coil starts to melt, producing a rush of water that overwhelms the drain pan in a short period, causing overflow and dripping from the indoor unit.
Signs this is your problem:
- Water leaks mainly when the system switches off, or shortly after
- Warm or reduced airflow from your vents, visible ice on the indoor unit or refrigerant lines
- The unit appears to be working but the room isn’t reaching the set temperature
What to do:
If you suspect a frozen coil, turn off your AC system to allow it to thaw completely. Place towels below the indoor unit to catch the water as it melts. Do not chip or scrape the ice off, as the coil fins are delicate and easily damaged.
Once fully thawed (typically one to three hours), check and clean the filters. If the coil refreezes after that, the underlying cause is likely low refrigerant or a blocked drain, both of which require a professional to diagnose properly. Running the system with a frozen coil can damage the compressor.
Get professional air conditioning
& electrical advice.
Cause 4: Damaged or Corroded Drain Pan
The drain pan sits beneath the evaporator coil and catches all the condensation the system produces. Over time, these trays can crack, corrode, or simply fill up if the drain line is partially blocked. A cracked or overflowing drip tray allows water to spill directly into the unit and surrounding structure, causing mould growth and water damage before you might even notice the problem.
Older air conditioners often develop cracks or corrosion in the drain pan. When the drain pan is damaged, water leaks before reaching the drain pipe. This problem is more common in systems older than 10 to 15 years.
Signs this is your problem:
- The unit is older (10-plus years) and leaking despite clean filters and a clear drain line
- Water appears to be coming from the bottom of the unit housing rather than overflowing from the top
- Rust-coloured staining around or below the indoor unit
What to do:
Inspect the drain pan for damage. Small cracks may be sealed with epoxy or waterproof sealant. Allow the air conditioner to dry before use, 24 to 48 hours. For larger issues, replacement of the drain pan may be recommended. If the drain pan is sold as part of the whole indoor unit chassis, replacement of the whole system is typically recommended.
For a licensed technician, replacing a drain pan is a straightforward job. If the system is already old enough that the drain pan is corroding, it’s also worth asking your technician whether the system overall is approaching the end of its economically useful life.
Cause 5: Low Refrigerant Levels
When the AC unit’s refrigerant gas levels drop, the evaporator coil may not be at the right temperature and often becomes too cold. This causes the coil to freeze, and when the ice melts excess water can overflow the drain pan or drip from the unit.
Low refrigerant is almost always the result of a leak somewhere in the sealed refrigerant circuit, not normal consumption. Since refrigerant issues involve the system’s internal components and pressure balance, it’s best to call a professional air conditioning technician to inspect and fix the problem. They’re also the only people who can legally handle refrigerant gases, which rules it out of being a DIY job.
Signs this is your problem:
- The system is cooling poorly alongside the water leak
- Ice is visible on the refrigerant pipes outside the unit as well as on the indoor coil
- A hissing sound near the indoor or outdoor unit
- The system runs constantly without reaching the set temperature
If you’re seeing all of the above together, refrigerant is the likely culprit rather than a drain blockage. This is a job for a licensed technician with an ARC (Australian Refrigeration Council) licence, and attempting to top up refrigerant yourself is illegal in Australia regardless of the products you might find at a hardware store.
Cause 6: Incorrect Installation or Unit Tilt
Air conditioners must be slightly tilted backward to allow water to drain toward the outlet. If installed level or forward-leaning, gravity works against the drainage system and causes water to build up inside the unit.
Poor installation can cause many problems, and your air conditioner leaking water is certainly up there. If the air conditioning unit is not level, condensation will not drain properly. If you have a poorly installed drain pipe or drain pan, that too will not handle the water flow properly.
Signs this is your problem:
- The unit has leaked from day one, or very soon after installation
- The leak doesn’t seem to correlate with filter condition or how hard the system is working
- The indoor unit appears to tilt forward, which can cause water to overflow and drip or come out through the air vents
What to do:
Ensure your unit is level. For split systems, check the indoor unit alignment. Do not adjust it yourself, contact a professional if it appears unlevel. If the system was recently installed and has been leaking since day one, contact the installer directly. A properly licensed installation should not produce this problem, and if the unit is still under warranty, the installation error should be corrected at no cost to you.
This is also a good reminder to only use licensed, experienced technicians for installation. An air conditioner installed by an unlicensed tradesperson has no warranty protection and no legal recourse if the installation is faulty.
Cause 7: Faulty Condensate Pump
Not all air conditioning systems can drain condensate by gravity alone. If your indoor unit is installed in a location where the drain can’t run downhill to an exit point, a condensate pump is used to push the water out. When the pump fails, water has no way to escape and simply backs up into the drain pan until it overflows.
This is more common in certain commercial installations, ceiling cassette units, or split systems installed in rooms where the drain outlet point is at the same height or above the unit.
Signs this is your problem:
- You can hear a small pump operating near the indoor unit normally, but it’s now silent
- Water is backing up and overflowing despite a clear drain line
- The unit is installed in a location where the drain runs level or slightly uphill
A faulty condensate pump is a straightforward replacement for a licensed technician. It’s not expensive and is usually diagnosed quickly.
Is a Leaking Air Conditioner Dangerous?
While the water itself is usually not harmful, leaks can cause electrical hazards, mould growth, or structural damage if left untreated.
The risks that actually matter:
Electrical hazard: Water and electrical components in close proximity are a genuine danger. If water is tracking toward your switchboard, inside the wall cavity, or into the unit’s electrical housing, switch the system off immediately and don’t run it until the leak is fixed.
Mould growth: The warm, humid environment of a wall or ceiling cavity with a slow water leak is ideal for mould. Mould can establish within 24 to 48 hours of a water event and, once inside a wall cavity, is expensive to remediate properly.
Structural damage: Prolonged leaks cause water damage to plasterboard ceilings, wall insulation, timber framing, and flooring. What starts as a minor drip can result in significant repair bills if left unattended for weeks or months.
The bottom line: Turn the unit off as soon as you notice water leaking inside your home and don’t restart it until the cause has been identified and addressed.
| Issue | DIY Possible? | Notes |
|---|---|---|
| Dirty/clogged filter | Yes | Clean every 4–6 weeks during heavy use. |
| Blocked drain line flush | Partially | Simple flush only; full clearing by technician. |
| Frozen coil (thaw and restart) | Yes | Turn off, thaw, check filter, restart. |
| Cracked drain pan (small crack) | Partially | Temporary sealant only; replacement by technician. |
| Low refrigerant | No | Licensed ARC technician required by law. |
| Faulty condensate pump | No | Technician replacement. |
| Incorrect installation tilt | No | Licensed technician. |
| Ducted system leak | No | Roof cavity access required. |
How to Prevent Your Air Conditioner Leaking Water in Future
Prevention is significantly cheaper than repair. Here’s the maintenance routine that keeps water leaks from becoming a regular problem:
Filter cleaning: Clean your filters every four to six weeks during heavy use periods. This single habit eliminates the most common cause of both frozen coils and drain blockages. If you haven’t been doing this regularly, our complete air conditioner cleaning guide covers exactly how to do it properly.
Drain flush: Flush the condensate drain line with a diluted white vinegar solution every three months during summer, more frequently if you’re on the Central Coast or have a particularly humid home. This prevents the algae and mould buildup that causes blockages.
Annual professional service: A licensed technician’s annual service includes checking refrigerant levels, cleaning the evaporator coil properly, clearing the condensate drain, and inspecting the drain pan for early signs of corrosion. Catching a developing crack or slow refrigerant leak during a service is far less expensive than dealing with the consequences of a leak left for a season.
Run the system efficiently: A system running efficiently at sensible temperature settings produces less strain on its drainage components than one constantly battling an unrealistic set point. Our guide on the best air conditioner temperature settings for energy saving covers how to keep the system running optimally through both summer and winter.
Keep the outdoor unit clear: While the outdoor unit isn’t directly involved in most indoor leaks, a unit surrounded by vegetation debris or without adequate clearance runs harder and is more prone to the refrigerant and coil issues that contribute to freezing and leaking.
Conclusion
An air conditioner leaking water inside your home is always worth taking seriously, even when the drip looks minor. The cause is almost always traceable and fixable, and the most common reasons, including a blocked drain line, dirty filter, or frozen coil, can often be partially addressed with basic DIY steps before a technician arrives.
The priority order is: switch the unit off to prevent further water damage, clean the filter, check whether you can flush the drain line, and if neither of those resolve it, or if you see any of the more serious signs (refrigerant symptoms, electrical concerns, ducted system leaks), get a licensed technician in promptly.
Most importantly, treat the leak as a prompt to establish a proper maintenance routine. A clean filter, a clear drain line, and an annual professional service will eliminate the vast majority of water leak risk for the life of your system.
Frequently Asked Questions
Why is my air conditioner dripping water inside the house?
The most common cause of an air conditioner leaking water inside is a blocked condensate drain line. When the drain pipe becomes clogged with dust or mould, condensation cannot drain properly. Water then builds up in the drain pan and leaks from the indoor unit. Other causes include dirty filters, frozen evaporator coils, and damaged drain pans.
Is it safe to run an air conditioner that is leaking water?
Running an air conditioner that is leaking water is not recommended, because it may damage electrical components or worsen the problem. Switch the system off until the cause is identified and fixed.
Can I fix a leaking air conditioner myself?
Some causes are DIY-friendly: cleaning a dirty filter, flushing the drain line with vinegar solution, or allowing a frozen coil to thaw. Others, including refrigerant issues, pump replacement, drain pan replacement, and ducted system leaks, require a licensed technician. When in doubt, turn the unit off and book a professional.
How do I stop my air conditioner from leaking water?
The most effective prevention is regular filter cleaning (every four to six weeks during heavy use), a quarterly drain flush with diluted white vinegar, and an annual professional service. These three habits address the vast majority of causes before they become leaks.
Why is my aircon leaking water when it's turned off?
If water is dripping from the indoor unit when the system is off, this is often a frozen evaporator coil thawing. The ice built up while the system was running and now melts when the system cycles off. Turn the power off fully, let it thaw, and check the filters. If this recurs, it points to a deeper airflow or refrigerant issue.
Is a small amount of water dripping from my outdoor unit normal?
Yes. A small, steady drip of water from the outdoor unit’s drain outlet during cooling operation is a completely normal sign that the system is working correctly and draining condensate as designed. Excessive water pooling around the outdoor unit, or water dripping from the indoor unit inside your home, is not normal.
How often should the condensate drain line be cleaned?
HVAC experts recommend cleaning condensate drain pipes at least once per year during routine servicing. For properties in humid coastal areas like the Central Coast, flushing every three months during summer is better practice to prevent the mould and algae growth that causes blockages.
Why is my ducted air conditioner leaking water?
Ducted system leaks are particularly concerning because the indoor unit and ductwork sit in the roof cavity, meaning a leak may not be visible until it has already caused significant ceiling or wall damage. Common causes include blocked condensate drains and cracked drain pans. If you notice water staining on your ceiling or around duct vents, switch the system off and book a technician promptly rather than waiting.
