Air Conditioner Leaking Water in Brisbane? 7 Causes and How to Fix Them

First, Understand Where the Water Comes From

A split system does not make water on purpose, but it does make a great deal of it. Warm, humid room air passes over an evaporator coil sitting well below dew point, and moisture condenses out of that air onto the cold fins — exactly the way a cold glass sweats on a Brisbane verandah in January.

That condensate runs down the fins into a drain pan beneath the coil, then out through a drain line that falls by gravity to a garden bed, a downpipe or a tundish. A wall-mounted split in a Brisbane summer can shed one to two litres an hour on a humid day. Every one of the faults below is a variation on the same theme: the water is still being made, but something is stopping it leaving.

This matters because it tells you what the problem is not. Water pouring out of the indoor head is not a sign that you need a regas. If you are also getting no cold air, that is a different fault — see our guide on an air conditioner running but not cooling.

1. Blocked Condensate Drain Line (the Brisbane Number One)

This is the cause on the clear majority of leaking-head callouts we attend, and Brisbane’s climate is the reason. The drain line carries a constant trickle of nutrient-rich water through a dark tube in warm, humid air — near-perfect conditions for biofilm. Algae, mould and dust bind into a slimy plug that narrows the line over a season or two until it stops passing water entirely.

Once the line blocks, the pan fills, overtops its lip, and the water finds the next way down: through the bottom of the indoor head, down the wall, or across the ceiling if the unit is a cassette or ducted return.

The tell-tale signs are a leak that gets steadily worse across a humid week, a gurgling noise from the indoor unit, and a drain outlet outside that stays dry while the unit runs. Find the drain line outlet — usually a small pipe discharging near the outdoor unit or into a garden bed — and watch it with the system running on Cool. If nothing comes out on a humid day, the line is blocked.

A licensed technician clears the line properly with a wet vacuum or nitrogen purge and then treats it to slow the biofilm returning. Blowing compressed air into the line yourself is a common DIY move that often fails: it can pack the plug tighter, or blow the blockage back into the pan.

2. A Dirty Filter That Has Frozen the Coil

This is the second most common cause and the one you can genuinely fix yourself. A clogged filter starves the evaporator coil of airflow. With too little warm air moving across it, the coil temperature falls below zero and the condensate freezes onto the fins instead of draining away.

You now have a block of ice sitting above a drain pan built for a trickle. When the unit cycles off — or when you switch it off in frustration — the ice melts all at once and dumps far more water than the pan can hold. That is why this fault so often shows up as a sudden gush rather than a steady drip.

Pull the front cover off the indoor head and lift out the mesh filters. If they are grey and furred, wash them in warm water, dry them completely and refit them. In Brisbane, filters want checking monthly through summer — pollen, dust and lint load up quickly when the unit runs daily.

If the coil behind the filters is already iced, turn the system to Fan only and let it thaw for two to three hours before running it again. If it ices up a second time with clean filters in place, stop and call a technician: you are most likely low on refrigerant, which is a sealed-system fault and a licensed job.

3. Cracked, Rusted or Silted Drain Pan

The drain pan sits under the coil catching everything the fins shed, and it does not last forever. Plastic pans go brittle with a decade of UV-free heat cycling and crack at the corners. Metal pans in older ducted systems rust through from the inside. Either way the water leaves before it reaches the drain.

Pans also silt up. Dust that gets past the filter settles into the pan and, mixed with condensate, forms a sludge that dams the outlet even when the drain line itself is clear. You can sometimes see this on a wall-mounted head by shining a torch up under the coil.

A silted pan can be cleaned during a standard air conditioner service. A cracked one needs replacing, and on some models that means partially dismantling the indoor head — worth getting quoted against the age of the system before you commit.

4. A Drain Line Without Enough Fall

Condensate drains by gravity alone on most residential splits. The line needs continuous fall from the pan to the discharge point — and if any section sags, runs level, or rises over a batten, water pools there instead of running out.

This is an installation fault rather than a wear fault, so the signature is different: the unit has leaked on and off since it was installed, or since a renovation moved the pipework. It is also why leaks often appear after a new unit goes in on a tight side passage where the installer had little room to work with.

A sagging line can sometimes be re-clipped to restore fall. A line that was routed uphill needs re-running, or a condensate pump fitting to lift the water to a drain point. If your system is newer and still under installation warranty, raise it with the original installer before paying anyone to re-pipe it.

5. Low Refrigerant

Low refrigerant lowers the pressure in the evaporator coil, which lowers its temperature, which freezes condensate to the fins — the same icing mechanism as a blocked filter, arriving from the opposite direction. The give-away is that the unit also cools poorly, and the ice returns quickly after a thaw even with clean filters.

Refrigerant does not get consumed. If the charge is low, it has leaked, and topping it up without finding the leak simply funds the same repair again next summer. That is a sealed-system job requiring an ARCtick licence — see our air conditioner leak repair guide and the regas cost guide for what that involves.

6. A Disconnected or Damaged Drain Hose

The flexible hose that joins the indoor head to the rigid drain line can work loose, perish, or be knocked off entirely — often by a tradesperson working in the ceiling or wall cavity, or by a rodent. Water then discharges straight into the wall or ceiling space, which is the worst version of this fault because you may see a stained ceiling long before you see a drip.

If your leak shows up as a damp patch or a sagging cornice rather than water running down the wall, treat it as urgent. Persistent moisture in a Brisbane ceiling cavity brings mould, and the make-good cost outruns the plumbing fix quickly.

7. Humidity That Simply Overwhelms the System

This one is not a fault at all, and it is worth knowing before you pay for a callout. During a Brisbane wet-season stretch — days of 85 % humidity, storms every afternoon — a system can condense more water than its drain was ever designed to shift, especially an older unit or one running with the doors and windows open.

The distinguishing feature is timing: the unit leaks only during the most humid few days of the year and behaves perfectly the rest of the time. Closing up the room, running on Cool rather than Dry, and having the drain professionally cleared before summer usually resolves it. If it leaks in September as readily as in February, it is one of the six faults above.

What You Can Do Yourself, and What Needs a Licence

Safe to do yourself: switch the unit off at the wall, place a towel and container to catch the water, remove and wash the filters, check the outdoor drain outlet for flow, and clear leaf litter away from the discharge point.

Call a licensed technician for: clearing a blocked drain line, cleaning or replacing a drain pan, re-running drain pipework, anything involving refrigerant, and any leak that has already reached plasterboard or a ceiling cavity. Handling refrigerant in Australia requires an ARCtick licence — it is not a DIY job, and unlicensed handling carries serious penalties.

What This Costs in Brisbane

A leaking indoor head is diagnosed as a standard repair attendance. A diagnostic starts from $150 to attend, find the fault and quote the fix — and that fee is credited toward the repair if you go ahead.

Where the leak turns out to be maintenance rather than a failure, a full air conditioner service from $149 covers the drain, pan and coil together and is usually the better value option — particularly if the system has not been serviced in over a year.

If the underlying cause is a refrigerant leak, pricing follows our leak repair cost guide rather than the figures above. All prices include GST.

The honest advice: if your system leaks every summer, the cheapest path is a pre-season service in September, before the humidity arrives and before every technician in Brisbane is booked out.

Frequently Asked Questions

Is it safe to run my air conditioner if it is leaking water?

No — switch it off. The water is landing on plasterboard, flooring or electrical fittings, and running the unit only produces more of it. Turning it off also lets a frozen coil thaw, which is often what is causing the overflow in the first place.

Why is my air conditioner leaking water inside but not outside?

Because the condensate is not reaching the drain. In a healthy system the water you would expect to see outside is exactly the water now coming out inside. A dry outdoor drain outlet while the unit runs on a humid day is the classic signature of a blocked drain line.

Can I clear a blocked air conditioner drain myself?

You can safely check the outlet for leaf litter and wash the filters. Clearing the line itself is best left to a technician with a wet vacuum — blowing compressed air into a drain line frequently packs the blockage tighter or forces it back into the pan, turning a simple clear into a strip-down.

Does a leaking air conditioner mean it needs a regas?

Usually not. Water leaking from the indoor head is condensate, not refrigerant — refrigerant is a gas and leaves no puddle. Low refrigerant can cause a leak indirectly by freezing the coil, but that comes with poor cooling as well. If the unit still cools properly, the problem is drainage.

How often should a Brisbane air conditioner have its drain cleared?

Once a year for most homes, timed for late winter or early spring so the system goes into summer clear. Brisbane’s humidity grows biofilm faster than cooler capitals, and homes near the river or with heavy tree cover often benefit from a clear every season.

My ceiling is stained under a ducted return — is that the air conditioner?

Very likely, and it is worth acting on quickly. A ducted system’s drain runs through the roof cavity, so a blocked line or disconnected hose discharges out of sight and shows up first as a stain, a sagging cornice or a musty smell. Have it inspected before the plasterboard needs replacing.