Diagnose the Problem First

Lift the cistern lid and observe:

  • Water flowing into the overflow tube — water level too high, float needs adjusting
  • Cistern draining slowly without using the overflow — flapper (rubber seal) not sealing
  • Hissing sound — fill valve worn and needs replacing

Fix 1: Adjust the Float (Water Level Too High)

  1. 1

    Identify your float type

    Old style: a ball on the end of a metal arm. Modern: a cup float that slides on the fill valve. Water level should be 2–3cm below the top of the overflow tube.

  2. 2

    Ball float: gently bend the arm downward

    A slight downward bend lowers the point at which the float shuts off the water. Flush and check — water should stop filling below the overflow tube.

  3. 3

    Cup float: turn the adjustment screw clockwise

    Find the adjustment screw or clip on the fill valve. Turn clockwise to lower the water level. Flush and check.

Fix 2: Replace the Flapper (Most Common)

  1. 4

    Confirm with the food colouring test

    Add a few drops of food colouring to the cistern. Wait 15 minutes without flushing. If colour appears in the bowl, the flapper is leaking.

  2. 5

    Turn off the water supply and flush

    The isolation valve is behind or under the toilet — turn clockwise to close. Flush to empty the cistern.

  3. 6

    Remove the old flapper and match it

    Unhook the flapper from the pegs on either side of the overflow tube and disconnect the chain from the flush lever. Take it to a hardware store to match exactly — flappers are not universal.

  4. 7

    Fit the new flapper and test

    Attach the new flapper, reconnect the chain (leave a little slack), turn on the water supply and let the cistern fill. Flush and repeat the food colouring test to confirm the seal.

CostA replacement flapper: $8–15. A complete fill valve and flapper kit: $20–40. A running toilet wastes 200–400 litres of water per day — fixing it immediately saves money on every water bill.

Frequently Asked Questions

A slow flapper leak wastes around 200 litres per day. A constantly running fill valve can waste 400+ litres per day. Over a year that is 70,000–150,000 litres — adding $150–400+ to your annual water bill in Australia. Fixing it pays for itself many times over.
This intermittent running (the toilet refills without being flushed) is almost always a slow flapper leak. Water drains slowly until the float drops low enough to trigger the fill valve. The food colouring test confirms it. Replace the flapper.