A toilet that runs on after the flush, hisses, or refills by itself in the night has a fault inside the cistern. It’s almost always one of two valves, and you can tell which in a few minutes with the lid off. A leak onto the floor is a different problem, and it’s covered further down.
This guide covers how the cistern works, how to find the part that’s failed, what you can safely adjust yourself, and when the job needs a plumber.
How does a push-button cistern work?
Two valves do the work. The inlet valve, with its float, lets water in and shuts off when the cistern is full. The outlet valve, under the flush button, holds that water back with a rubber seal until you flush. An overflow tube carries any excess into the bowl, so a fault ends up in the pan, not on the floor.
That gives water two ways into the bowl when nobody has flushed:
- Over the top of the overflow tube, when the inlet valve doesn’t shut off in time.
- Past the outlet valve seal, when the rubber has worn or picked up grit.
Older cisterns may have a lever and a float on an arm (a ball cock), and US guides talk about a flapper and chain. The same two faults apply; only the parts look different.
Where is the water going?
Start with what you can see and hear. Water in the bowl means a valve inside the cistern. Water on the floor, behind the cistern or between the cistern and pan means a seal or connection outside it. The table sorts the common signs.
| What you notice | Most likely cause | Next step |
|---|---|---|
| Constant hiss; water spilling into the overflow tube | Inlet valve not shutting off, or the water level set too high | Lift the float to test, then adjust the level |
| Silent trickle into the bowl; level below the overflow | Worn outlet valve seal | Dye test, then replace the seal |
| Cistern refills for a few seconds on its own, often at night | Outlet seal letting water out slowly | Dye test |
| Runs on after a flush until you jiggle the button | Button or outlet valve not resetting | Check the button springs back up |
| Slow refill or a weak flush | Isolation valve part-closed, a blocked inlet valve, or the level set too low | Open the isolation valve fully, then check the level |
| Water between the cistern and pan, often when flushing | Perished seal or bolt washers between them | Plumber |
| Drip under the cistern at the wall | Flexible supply hose or its fittings | Turn off the isolation valve; plumber |
| Water at the base of the pan | Pan connector or pan seal | Plumber; avoid using that toilet |
| A stain down the bowl where the water runs | A long-running slow leak | Dye test |
How do you tell which valve has failed?
Lift the lid and look at the overflow tube. If water is running over its top, the inlet side is at fault. Lift the float by hand: if the water stops, the float needs lowering; if it keeps running, the inlet valve itself needs a new seal or replacing. If the level is below the overflow, test the outlet seal with food colouring.
The dye test. Watercare suggests it in its own leak advice. Put a few drops of blue or green food colouring into the cistern and don’t flush. Waipā District Council’s guidance is to wait 15 minutes; if the bowl water has changed colour, water is getting past the outlet valve. Tiaki Wai (formerly Wellington Water) notes that a slow leak can take an hour or two to show, so a clear bowl after 15 minutes isn’t the final word. Flush afterwards to clear the colour.
A second check. Turn off the isolation valve, the small tap on the pipe beside or behind the toilet, and mark the water level inside the cistern with a pencil. If the level drops while nothing is coming in, the water is leaving through the outlet seal.
Why does the toilet refill by itself, or keep running after a flush?
A cistern that tops itself up for a few seconds with nobody near it is losing water past a worn outlet seal: the level drops until the float opens the inlet valve, which refills it. One that keeps running after a flush usually has a button or outlet valve that hasn’t reset, so water flows straight through.
Press the button again and watch it. It should spring back up fully; if it sticks down, the outlet valve stays open and the inlet valve can’t catch up.
The reverse problem, a cistern that fills slowly or not at all, is usually on the supply side. Check the isolation valve is turned on fully, since a part-closed one gives a slow fill and a weak flush. If it is, Fluidmaster’s instructions for one common inlet valve say that a valve which won’t turn on or off, or refills slowly after being in use for some time, may need a replacement seal. Grit from the supply can also lodge in the valve.
How do you set the water level?
Most cisterns have a water line moulded or marked inside; the water should stop there. One outlet valve manufacturer specifies a maximum level no higher than 20 mm below the top of the overflow tube. Most modern inlet valves have an adjustment rod or screw on the float: moving the float down shuts the water off sooner.
Make small changes, flush, and let the cistern refill before you check again. Too high and water runs into the overflow and the inlet valve never closes. Too low and the flush is weak, which can look like a blockage. If the bowl rises and drains slowly rather than the cistern running, that’s a different problem, covered in our guide to a blocked toilet.
Older ball cock valves have a float on a metal or plastic arm, adjusted with a screw where the arm meets the valve. If the valve or arm is corroded or damaged, adjusting it won’t hold, and the valve needs replacing.
Can you replace the seals yourself?
It depends on the job. The Plumbers, Gasfitters and Drainlayers Board’s consumer advice lists what homeowners may do, including “replace or repair taps, ball cocks and plugs”, and what they may not: fixing or unfixing any plumbing fixture, or any pipe that supplies water. If you’re unsure where your job falls, ask the Board before you start.
If you do take it on, a few things make the difference:
- Match the part to the cistern. Inlet and outlet valve seals are made for particular brands and valve designs. Bunnings NZ, for example, sells washer kits made to suit specific brands’ cisterns. Take the old seal, or a photo of the valve and any brand name on it, when you buy the new one.
- Isolate and empty first. Turn off the isolation valve and flush to empty the cistern. On many push-button cisterns, the outlet valve then twists or unclips from its base so the seal can be changed.
- In-wall cisterns sit behind the wall with only the flush plate showing. Some are designed to be serviced through the opening behind the plate, with no tiles removed. Know your model before you start.
- Skip bleach tablets. Fluidmaster warns against drop-in cleaners containing bleach or chlorine, which it says damage cistern components and may cause flooding.
If you rent, tell your landlord or property manager rather than fixing it yourself. Watercare’s leak guidance says landlords are responsible for fixing leaks on their property, and asks tenants to report them quickly because they’re paying for the water. For a tap that won’t stop dripping, our guide on why a tap keeps dripping covers washers and cartridges.
What if the water is on the floor, not in the bowl?
Water outside the toilet comes from one of three places: the seal between the cistern and pan, the flexible hose that feeds the cistern, or the connection between the pan and the drain. Dry everything with a towel, lay dry paper towel around the base and behind the cistern, and wait about half an hour to see which gets wet first.
- Between the cistern and pan. On a close-coupled suite, where the cistern sits on the back of the pan, a rubber seal and bolt washers join the two. When they perish, water shows there, often only during a flush.
- At the supply hose. A drip at the wall or under the cistern is usually the flexible hose or its fittings. Turn the isolation valve off until it’s replaced.
- At the base of the pan. That’s the pan connector or seal between the toilet and the waste pipe. Water under the floor gets expensive if it’s left, and the toilet has to be lifted to fix it, which is work for a licensed plumber.
A pan that rocks or moves on the floor is worth having checked too.
How much water does a running toilet waste?
It depends on the fault. Tiaki Wai puts a slow-leaking toilet at up to 28 litres a day. Waipā District Council says a continuously running toilet can waste more than 700 litres a day, and notes that a toilet leak is often silent. Either way, the water runs straight down the pan.
On an Auckland bill it counts twice. Watercare charges wastewater on the water your meter records, so water lost down the toilet adds to both lines.
To check whether you have a leak anywhere, Watercare suggests a meter test: read the meter, use no water for at least four hours, and read it again. If it has moved, something is leaking. Watercare’s leak allowance terms define a leak as water escaping from “any pipes, fittings or devices on your side of the point of supply”, so if a high bill has already arrived, ask Watercare whether the allowance applies. Our article on water leaks and who pays explains how the allowance works.
Should you repair the toilet or replace it?
A worn seal or a tired inlet valve is a cistern part, not a reason for a new toilet. Replacement makes sense when the pan or cistern is cracked, since water coming through the ceramic itself can’t be sealed reliably, or when the suite is badly stained, an old water-hungry model, or part of a bathroom refresh.
Most modern suites fit the existing waste and water connections, so a new toilet doesn’t mean opening walls or floors.
What we do when you call us about a running or leaking toilet
We find which part has failed first: the inlet valve, the outlet valve seal, the seal between cistern and pan, or the pan connector. Inlet and flush valves are replaced inside the cistern, and leaks at the base are fixed at the connector and floor flange.
If the suite is sound, we repair the internals and it’s good for years. If replacing it makes more sense, we tell you, and we can supply and fit a new suite or fit one you’ve bought. Either way, you hear the options and the price before we start. We connect the water with an isolation valve if there isn’t one, set the pan level, and test it through several flushes before we leave.
Our toilet repairs page covers the work in more detail.