The short answer
A toilet keeps running when water either leaks from the cistern into the bowl past a worn outlet seal, or keeps filling because the inlet valve won't shut off and the water spills down the overflow. A flush button that sticks does the same. Food colouring in the cistern shows which one it is.
We've been repairing toilets and cisterns across Melbourne's northern suburbs since 2012, and we quote the job free with no call-out fee.
A running toilet is easy to stop noticing. The faint hiss becomes part of the house, like the fridge. Then the water bill arrives and it gets noticed again, all at once.
The good news is there are only four usual causes, and a five-minute test tells you which one you’ve got.
| What you notice | Most likely cause |
|---|---|
| A ripple or trickle at the back of the bowl | Worn or dirty outlet seal |
| Cistern refills by itself every so often | Worn or dirty outlet seal, leaking slowly |
| Water running down the overflow tube | Inlet valve not shutting off, or float set too high |
| Runs until you jiggle the button | Sticking flush button |
| Hissing that never stops | Inlet valve not shutting off |
How a toilet cistern works
Most Australian toilets have a dual-flush cistern with two main parts inside. The outlet valve sits at the bottom and lifts when you press the button, letting water rush into the bowl. When it drops back down, a rubber seal closes it off.
The inlet valve refills the cistern. A float rises with the water level and shuts the valve off when the cistern is full. If anything goes wrong with either valve, the toilet keeps running.
There’s also an overflow, built into the outlet valve. If the cistern overfills, the excess runs down it into the bowl rather than spilling onto your floor. That’s why an overfilling cistern shows up as a running toilet and not a flood.
The food colouring test
Take the lid off the cistern and put in a few drops of food colouring. Don’t flush. Leave it for 15 minutes, then look in the bowl.
If the colour has come through into the bowl, water is getting past the outlet valve. Look at the water level in the cistern too. If it’s up at the top of the overflow and trickling down it, the inlet valve isn’t shutting off.
Cause 1: a worn or dirty outlet seal
This is the most common one. The rubber seal at the bottom of the outlet valve hardens and wears over time, and a speck of grit or a bit of mineral build-up is enough to stop it sealing. Water trickles past into the bowl, the level in the cistern drops, and the inlet valve tops it up. Round and round, all night.
Turn off the stop tap to the cistern and flush to empty it. Most outlet valves in dual-flush cisterns lift or twist out. Wipe the seal and the surface it sits on clean. If the seal is hard, cracked or misshapen, it needs replacing, and the part has to match your cistern’s brand.
Cause 2: the inlet valve won’t shut off
If water is running down the overflow, the cistern is overfilling, and the inlet valve isn’t closing when it should. Grit caught in the valve can hold it open, or the valve itself can wear out.
You’ll usually hear this one. A hiss that never stops, even when nobody’s flushed for an hour, is the inlet valve still letting water in. A worn inlet valve gets replaced, which means disconnecting it from the water supply.
Cause 3: a sticking flush button
Sometimes the button doesn’t come all the way back up after a flush, and it holds the outlet valve open. The giveaway is a toilet that runs until somebody jiggles the button, then stops.
Take the lid off and watch the button mechanism as you flush. Buttons stick when the rods or cables that link them to the outlet valve are bent, misaligned or catching on something. Freeing them is often a five-minute job.
Cause 4: the float is set too high
If the float is set too high, the inlet valve keeps filling past the top of the overflow, and the extra water runs down it into the bowl. Nothing’s broken. It’s just adjusted wrong.
Most modern inlet valves have an adjustment screw or a clip on the float. Set it so the water stops a couple of centimetres below the top of the overflow. There’s usually a water level mark inside the cistern as a guide.
What you can fix, and what you can’t
A running toilet wastes water around the clock. Yarra Valley Water says unnoticed leaks can add between $300 and $1,200 a year to a household’s water costs, and a toilet leaking into the bowl is one of the easiest to miss.
| Fix | Do it yourself? |
|---|---|
| Freeing a sticking flush button | Yes |
| Adjusting the float | Yes |
| Cleaning grit off the outlet seal | Yes |
| Replacing the outlet valve seal or assembly | Often, if you can match the part |
| Replacing the inlet valve | No, it means disconnecting the water supply |
| A leak at the base of the pan or from the stop tap | No, that’s licensed work |
Anything that means disconnecting the water supply to the cistern is licensed plumbing work in Victoria. So is a leak at the base of the pan, or water on the floor around it, which can mean a failed seal at the connection to the sewer.
If you’d rather not spend a Saturday with your arm in the cistern, our toilet and cistern repairs page covers what we do. And if there’s a tap dripping in the same bathroom, our guide on how to fix a leaking tap walks through that one. Want to know what a call-out costs before you book? Our guide to what a plumber costs in Melbourne has the current rates.
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