A leaking aircon is almost always caused by one of four things: a blocked condensate drain, a frozen evaporator coil, a damaged drain pan, or a poorly installed unit. The first one accounts for the majority of service calls we get on this. The fix is usually quick, but ignoring it can turn a $200 service call into a five-figure ceiling repair.

Here’s how to work out which one you’re dealing with, what you can fix yourself, and when to get a tech out.

What’s normal and what’s not

Worth covering briefly because it confuses people.

Your aircon produces water as part of normal operation. The evaporator coil inside the indoor unit gets cold, moisture from the room air condenses on it, and that water drains away through a small pipe. On a humid Sydney summer day, a healthy ducted system can produce 5 to 10 litres of water in a few hours. That’s normal.

What’s not normal: water dripping from the indoor unit into the room, water staining the ceiling under a ducted indoor unit, water pooling under the outdoor unit’s drain pipe at the wrong rate, or ice forming on the indoor coil or copper pipes.

If you’re seeing any of those, here’s what’s happening.

Cause 1: blocked condensate drain (the most common)

The drain line is a small PVC pipe that carries condensate water away from the indoor unit. Over time, algae, dust and biofilm build up inside it. Once the line blocks, water has nowhere to go, backs up into the drain pan, overflows, and ends up running down the wall (split system) or staining the ceiling (ducted).

How to tell: water dripping from the bottom of the indoor unit, water marks on the wall directly under a wall split, or brown staining on a hallway ceiling under a ducted indoor unit.

The fix: a tech can usually clear the line in 10 to 15 minutes using a wet/dry vacuum on the outdoor end of the drain. Costs $150 to $250 for the call-out. Some homeowners attempt this themselves with a shopvac, which works fine if you can find both ends of the drain line and apply suction at the right end. If you’re not sure, get a tech.

The DIY prevention: during your quarterly homeowner maintenance, pour a cup of distilled white vinegar into the access point on the drain line (usually a T-piece near the indoor unit). It kills the algae before it builds up enough to block the line.

Don’t ignore this one. A backed-up drain that floods a plasterboard ceiling is a five-figure repair. We’ve replaced ceilings in homes across Sydney that started with a $0 maintenance job nobody got around to.

Cause 2: frozen evaporator coil

If you can see ice on the indoor coil, on the copper pipes leading out of the indoor unit, or on the outdoor unit, the system has frozen up. When that ice melts (usually after the system is switched off for a few hours) you get a flood of water that overwhelms the drain.

Two things cause this:

Restricted airflow, which usually means a clogged filter. A genuinely matted filter starves the coil of warm room air, the coil temperature drops below freezing, and condensation freezes on the coil instead of draining away.

Low refrigerant. The system runs at lower pressure than designed, the coil temperature drops below freezing, and the same thing happens. Refrigerant doesn’t get used up over time, so if it’s low, there’s a leak somewhere in the system.

How to tell which: clean the filter first. Switch the system off for 4 to 6 hours to let any ice fully thaw. Run it again. If the freezing returns within a day or two with a clean filter, it’s almost certainly a refrigerant leak.

The fix: filter clean is free. Refrigerant repair is $300 to $800+ depending on where the leak is and how much refrigerant needs replacing. Topping up without finding the leak is against industry best practice and will leave you with the same problem in a few months. Refrigerant work requires an ARC-licensed technician under federal law.

Cause 3: cracked or rusted drain pan

The drain pan sits under the evaporator coil to catch condensate before it goes into the drain line. On older systems (typically 12+ years), the pan can crack (plastic) or rust through (metal). Water bypasses the drain entirely and falls straight through the bottom of the indoor unit.

How to tell: the drain line itself is clear (no blockage), the filter is clean, the coil isn’t frozen, but water still appears under the unit. A tech can confirm by removing the cover and inspecting the pan directly.

The fix: drain pan replacement, $200 to $500 depending on system. On systems past 15 years old, a cracked pan is often a sign other components are aging too, and worth getting a replacement quote alongside the repair to compare options.

Cause 4: poor installation

If the system has been leaking since installation, or started leaking shortly after, the install is the likely cause. We see this frequently when called out to systems installed by quick-and-cheap operators.

The common faults: indoor unit not levelled correctly so water flows to the wrong end of the drain pan, drain line installed with insufficient fall (water doesn’t flow downhill, so it sits in the line until something backs up), refrigerant pipe insulation taped together loosely so condensate forms on the pipes inside the wall, or condensate pump that’s failed or wasn’t fitted where it should have been.

The fix: depends on the specific issue. Most can be corrected, but it’s a tech job. Worth pushing back on the original installer if the system is still under workmanship warranty. Most reputable installers offer 5 to 7 year workmanship warranties on the install itself.

Sydney-specific factors worth knowing

A few things that come up regularly across our service area.

Humidity matters. Sydney’s late-summer humidity (February and March especially) puts more moisture through the system than drier months. Drains that coped fine through January start blocking when the load increases. If your system has leaked once, get the drain cleared properly before next summer rather than waiting for it to happen again.

Apartment installs are particularly vulnerable. Body corporate restrictions in inner-west and eastern suburbs apartments often force drain runs with poor fall, and a leak that damages a unit below yours becomes your problem to fix. Worth getting drain lines flushed annually rather than waiting.

Western Sydney dust loads filters faster. Penrith, Liverpool, Campbelltown and similar areas have noticeably more airborne dust than coastal suburbs, so filters need cleaning more often. Frozen coils from clogged filters are a more frequent cause of leaks out here.

What you can do yourself

Three things, all genuinely homeowner-level:

Clean the indoor unit’s filter every 4 to 6 weeks during summer. Pop the front panel, slide the filter out, vacuum and rinse. Let it dry fully before reinstalling.

Pour a cup of white vinegar down the drain access every 3 to 4 months. Kills the algae that causes blockages.

Check around the outdoor unit’s drain pipe for water flow during operation. A trickle is normal. No flow on a humid day suggests a blockage somewhere.

When to get a tech out

Call a technician if any of these are true: water is staining the ceiling, water keeps appearing after a filter clean, ice is forming on the coil or pipes, the smell coming from the unit is musty or burnt, or the system is over 12 years old and leaking for the first time.

The longer you leave a leak, the more expensive it gets. A blocked drain caught early is a $200 fix. The same drain ignored for three months can flood a ceiling.

Frequently asked questions

Can I just put a bucket under it until I get it fixed? For a couple of days while you’re booking a tech, sure. Beyond that, water is finding its way somewhere you can’t see. Get it sorted.

Why does my aircon only leak on really humid days? The system produces more condensate on humid days, which can overwhelm a partially blocked drain that copes fine on dry days. Get the drain flushed properly.

Is a leaking air conditioner dangerous? Not directly, but the secondary effects can be expensive: water damage to plasterboard, mould growth in walls and ceilings, and electrical components getting wet. Not something to leave alone.

How often should drain lines be cleared? A professional flush as part of an annual service is enough for most systems. Apartments and homes with previous drain issues benefit from twice-yearly attention.