Why Your Toilet Cistern Keeps Running, and How to Stop It

What is actually leaking

Strip away the noise and a cistern is a bucket with a tap at the top and a plug at the bottom. Water can only leave it by 2 routes, which is why diagnosis is far simpler than it feels standing over it at 6am.

Route 1 is the overflow tube, the vertical standpipe rising out of the middle of the cistern. Its whole job is to send water safely into the bowl instead of over the rim onto your floor if the cistern overfills. Water running into the overflow means the inlet valve is not shutting off, either because the float is set too high or because the valve itself has failed. The cistern is full, the inlet has not noticed, and the surplus goes down the standpipe.

Route 2 is the outlet at the base, where the flush valve seals against the seat. Water leaking past that seal drains quietly into the bowl. The level in the cistern drops, the float falls, the inlet valve opens to top it up, and the cistern refills. That is why an outlet leak is heard as a cistern that cycles on and off with nobody near it, or as a permanent faint trickle in the pan.

• Constant loud rushing and water visibly running into the overflow tube: inlet valve or float height.
• Cistern refills for 20 or 30 seconds every few minutes with nobody using it: outlet seal.
• Faint permanent ripple across the water in the bowl: outlet seal, small leak.
• Hissing that gets louder over hours then stops when you jiggle the button: linkage or button binding, holding the valve open.
• Water on the floor rather than in the bowl: this is not a running cistern. Stop and look at the pan connector, the inlet fitting and the cistern itself for a crack.

The dye test: find it in 20 minutes with no tools

Before removing a single part, do this. It costs nothing, it takes 20 minutes of waiting, and it tells you which of the 2 faults you have with total certainty. Plenty of people replace an inlet valve for a problem that was always the outlet seal, and the dye test is what stops that.

1. Lift the cistern lid and set it flat on a towel, well away from where you are working. Vitreous china lids chip easily, and a replacement lid for a discontinued suite is often harder to find than a whole new cistern.
2. Wait until the cistern has finished filling and is sitting still.
3. Add a few drops of dark food colouring to the water in the cistern. Blue or green shows up best against white china. A dye tablet from a plumbing supplier does the same job.
4. Do not flush. Leave the toilet completely alone for 20 minutes.
5. Come back and look into the bowl. Coloured water in the bowl means water is passing the outlet seal at the base of the cistern.
6. While you are there, look at the overflow standpipe. If the water level in the cistern is at or over the top of it, the inlet valve is the fault instead, or as well.

The 2 faults can coexist, and on a cistern that has never been serviced they often do, because both rubber parts are the same age and have lived in the same water. If the dye test is positive and the level is high, plan on a full seal kit rather than a single part.

Australian cisterns are not the ones in most online guides

Search this problem and most of what comes back describes a rubber flapper hinged along its back edge, lifted by a chain from a trip lever on the front of the cistern. That is an American single-flush design. It exists in Australia, on older single-flush cisterns and on some imported suites, but it is not what is in the majority of Adelaide bathrooms, and following those instructions on an Australian cistern gets confusing fast.

Dual flush has been the Australian standard since the 1980s, and the dominant local design uses a drop valve: a vertical plastic tower sitting over the outlet, carrying a large flat rubber seal on its base, lifted by the push buttons through a short rod or a cable. Caroma, Fowler, Stylus and Clark cisterns are all built around variants of it. There is no chain and no flapper.

• The part you are looking for is a drop valve seal or outlet valve washer, not a flapper. Ask for it by that name and the counter will know what you mean.
• Half flush and full flush volumes are set by an air chamber or an adjustable collar on the tower, not by chain length. A dual flush cistern that dumps its whole contents on the half flush usually has that collar out of position or an air chamber that has cracked.
• The buttons act on the tower through a rod whose length is adjustable. A rod set slightly too long holds the valve fractionally open after every flush, which is a very common cause of a permanent trickle and is free to fix.
• Most towers release with a quarter turn, in the same way a bayonet light fitting does. If it does not want to come out, it is almost always turning the wrong way rather than needing force.

Concealed in-wall cisterns are increasingly common in Adelaide bathroom renovations, particularly in the inner south where a small original bathroom is being reworked to feel larger. Geberit is the most frequently installed brand. Everything on these is designed to be serviced through the flush plate opening, and the plate is meant to come off by hand or with a light pull at the clips. Never cut the wall. If the plate will not release, identify the model first, because each manufacturer has its own release method and levering at the wrong point cracks the plate.

Fixing an inlet valve that will not shut off

Start with the free fix. The water level should sit roughly 20mm to 25mm below the top of the overflow standpipe. If it is higher, adjust the float down using the screw, clip or collar on the inlet valve, in small increments, flushing and letting it refill between adjustments. Plenty of running cisterns are cured right there, because the level has crept up as the valve wore.

1. Close the isolation tap on the inlet pipe below the cistern. Most are a slotted screw head turned with a flat screwdriver: the slot lines up with the pipe when open and across it when closed.
2. Flush, and sponge out whatever water remains in the base.
3. Unscrew the top cap of the inlet valve and check the diaphragm and the small inlet filter behind it. Grit and scale on either will hold the valve fractionally open forever.
4. Check the float itself. A float that has taken water inside will never rise far enough to shut the valve, and it feels noticeably heavy when you lift it. It gets replaced, not repaired.
5. If the valve still will not seal with a clean diaphragm and a sound float, replace the whole inlet valve. Undo the supply nut, undo the locking nut under the cistern, lift the old one out and fit the new one at the same height.
6. Set the refill tube so it discharges into the overflow standpipe but does not sit below the water line, then reopen the isolation tap slowly and watch for drips at the base and the supply connection.

A pair of Adelaide-specific things shorten the life of inlet valves here, and both are worth knowing before you fit the replacement. The first is what is in the water. Adelaide's supply blends local reservoir water with River Murray water, and the proportion shifts through the year depending on rainfall and pumping. It carries enough dissolved mineral and fine sediment to scale a diaphragm and to block an inlet filter, and after any mains work or a hydrant flush in your street a slug of grit gets pushed into every house on the line. That is why a cistern sometimes starts running within a day or 2 of council or SA Water work outside.

The second is pressure. AS/NZS 3500 limits static pressure at fixture outlets in new work to 500 kPa, and plenty of older Adelaide homes have never had a pressure limiting valve fitted. Properties along the hills face, fed from higher reservoirs, commonly sit well above that figure. High inlet pressure makes a valve chatter, hammer and wear its diaphragm out early, and it does the same to the flexible hoses under every vanity in the house. If you are replacing inlet valves more often than every few years, the valve is the symptom. A pressure limiting valve at the meter is the fix, and it protects the hot water unit and the tapware at the same time.

Fixing an outlet seal that leaks past

This is the more common of the 2 faults and the more satisfying to fix, because it is a small part and the difference is immediate.

1. Close the isolation tap, flush the cistern and sponge the base dry.
2. Unclip the buttons or unscrew the button surround, and disconnect the rod or cable from the top of the drop valve tower.
3. Twist the tower a quarter turn and lift it clear. Note which way it came out before you put anything down.
4. Peel the old rubber seal off the base of the tower. Take it with you when you buy the replacement, along with a photo of the whole cistern internals and the brand name stamped inside the lid or on the china.
5. Clean the plastic outlet seat in the cistern base with a cloth and white vinegar to lift the scale. Never use sandpaper, a scourer or a blade: a scratched seat will never seal again, and at that point the whole flush valve body has to be replaced.
6. Fit the new seal, seat the tower back down and lock it with a quarter turn, reconnect the rod and set its length so the buttons return fully to rest.
7. Reopen the isolation tap, let the cistern fill, and run the dye test again to confirm the leak has stopped.

Take the old parts to a plumbing supplier rather than a hardware aisle. Cistern internals are brand and generation specific, the packaging photographs all look identical, and a seal that is 2mm out in diameter seals nothing at all. The counter staff at a trade supplier identify these by eye every day.

How much water a running cistern wastes

This is worth doing as arithmetic rather than as a scare, because the volumes are their own argument. SA Water bills residential customers quarterly and charges by the kilolitre on a tiered scale, so the further into the quarter a leak runs, the more each additional litre matters.

To confirm the loss is real and is coming from the toilet, use the meter. Turn off every tap and appliance in the house, then read the leak indicator on the water meter: on most Adelaide meters it is a small red or black triangle, star or disc that spins on the smallest flow. Note the position, wait 30 minutes without using any water, and read it again. Movement means water is leaving the property somewhere. Then close the isolation tap under the cistern and repeat. If the indicator stops, you have found it.

If the indicator keeps moving with the toilet isolated, the toilet is not your only problem, and the next step is a proper look at the whole property rather than another cistern part.

When to stop and call a plumber

Being honest about this matters more than winning the job. Most running cisterns are a seal kit, a screwdriver and 20 minutes, and a competent adult with the right part will do it properly. The list below is where the job stops being a home repair.

• Do it yourself: adjusting float height, replacing an inlet valve, replacing a drop valve seal, freeing a binding button rod, cleaning a scaled inlet filter.
• Call a plumber: the outlet still leaks after a new seal, because that means the plastic seat is scored or warped and the flush valve body needs replacing.
• Call a plumber: the isolation tap under the cistern will not close, or the head snaps when you try. Now the whole property has to be isolated to do anything.
• Call a plumber: a concealed in-wall cistern where the flush plate will not release, or where the fault is behind the frame.
• Call a plumber: water appearing on the floor behind the pan. That is usually the pan connector or the outlet rather than the cistern, and it is a sewer connection, not a supply problem.
• Call a plumber: a hairline crack in the china, anywhere. Cracked cisterns fail without warning and they fail full.

The other genuine limit is age. On a suite older than about 25 years the internals are frequently discontinued, and no amount of hunting produces a seal that fits. At that point replacing the cistern, or the whole suite, is the honest answer. When we get there, Marc says so with the old parts in hand and explains the reasoning before anything is ordered.

Allmat handles toilet repairs across the inner south and the Adelaide Hills from a base at Westbourne Park. Marc is a licensed plumber and gas fitter with 20 years of experience, the price is discussed with you before any work starts, and he texts an arrival time before setting off so nobody spends a day waiting on a toilet repair.

Questions we get asked

References

• Water Rating (WELS), Australian Government: fixture water efficiency ratings
• SA Water: residential tariffs, meter reading and leak indicators
• Australian Building Codes Board: NCC Volume 3, the Plumbing Code of Australia