12 Jun 2026
That faint hiss of water long after a flush is easy to ignore — until the water bill arrives. A constantly running toilet can waste hundreds of litres a day, all of it disappearing silently down the bowl. Most cases come down to a few worn parts inside the cistern. Here is what causes it and what to check.
A worn or misaligned flush valve
The rubber flush valve, or flapper, at the bottom of the cistern seals water in the tank between flushes. Over time it hardens, warps or shifts, letting water seep into the bowl continuously. A worn valve is the single most common cause of a running toilet — and usually a simple replacement.
A faulty fill valve or float
After you flush, the fill valve refills the tank and shuts off when the float reaches the right level. If the float sits too high or the valve fails to close, water keeps flowing into the overflow tube and never stops. Adjusting the float or replacing the fill valve resolves it.
The water level is set too high
If water is trickling into the overflow tube, the level in your cistern is too high. Lowering it to the marked fill line stops the constant overflow and uses less water per flush.
A quick test to confirm a leak
Not sure if your toilet is leaking? Try this simple dye test:
- Add a few drops of food colouring to the water in the cistern
- Do not flush — wait about 15 to 20 minutes
- Check the bowl: if coloured water has appeared, the flush valve is leaking
When to call a plumber
Many running-toilet fixes are minor part replacements, but a cracked cistern, a leak at the base of the toilet, or repeated faults after a repair point to a bigger problem. A plumber will fit the right parts, check the whole flush system and make sure the leak is gone for good — so you stop paying for water you never use.
A running toilet only gets more expensive the longer it is left. If yours will not stop, getting it checked early is the cheapest fix of all.