Skip to content

Is my toilet leaking if the water level moves?

The short answer

A continuously running toilet is a leaking outlet valve or an incorrectly set inlet valve, and it can waste a very large volume of water invisibly, which is a common cause of an unexplained bill.

TradieAnswers editorial · written in-house and sourced to the authorities listed below

Checked against 4 published sources, listed in full at the end of this page. How we source this

A toilet leaking into the bowl produces no puddle, makes little noise once it settles, and can run continuously for months. Because it drains straight to sewer, the only evidence is the bill, which is why it is the first thing to check when a bill jumps with no change in habits.

The pencil test

  1. Mark the water level in the bowl, or drop food colouring into the cistern.
  2. Do not use the toilet for thirty minutes.
  3. If the level in the bowl has changed, or colour has appeared in the bowl, the outlet valve is passing water.

What usually needs doing

  • The outlet valve seal, which perishes and is the most common cause.
  • The inlet valve, if the level is set above the overflow so water runs down it continuously.
  • The whole cistern mechanism, which on older suites is often cheaper to replace as a unit than to source parts for.

Worth knowing: replacing a whole toilet suite in Australia means the replacement must meet the water efficiency labelling requirements, so an old single flush suite cannot be replaced like for like.1

Sources

Every source below was fetched and its response recorded on the date shown. Where a government site blocks automated clients, that is noted rather than reported as a successful check.

  1. Water Rating (WELS) · Australian GovernmentChecked 2026-08-10 · responded 200 OK
  2. NCC 2022 Volume Three: Plumbing Code of Australia · Australian Building Codes BoardChecked 2026-08-11 · responded 200 OK
  3. Backflow prevention · Sydney WaterChecked 2026-08-10 · responded 200 OK
  4. Building and Plumbing Commission · Building and Plumbing Commission VictoriaChecked 2026-08-10 · live, but blocks automated clients so the specific page could not be confirmed

Read next

More plumbing questions

All 31 →