Why Does My Toilet Keep Running?

That constant hissing or trickling sound coming from your toilet cistern is more than just annoying — it's money draining straight down the drain. A running toilet can waste anywhere from 200 to over 1,000 litres of water per day depending on the severity of the leak. In NSW, where water isn't cheap and every household is encouraged to be mindful of usage, this is a problem worth fixing promptly.

The good news is that running toilets are one of the most common plumbing issues we deal with, and in most cases they're very straightforward to diagnose and fix.

How Does a Toilet Cistern Actually Work?

Understanding the problem is easier once you understand the basic mechanics. When you flush, water drains from the cistern into the bowl via a flush valve (the seal at the bottom of the cistern). The cistern then refills via an inlet valve (sometimes called a ballcock or fill valve). When the cistern is full, the inlet valve shuts off automatically. That's the whole cycle.

A running toilet means part of that cycle isn't working properly — either water is leaking out of the cistern, or the inlet valve isn't shutting off correctly.

Common Causes

1. Worn or Warped Flush Valve Seal

The most common cause of a running toilet in Australian homes. The flush valve seal (sometimes called the flapper in older-style cisterns, though most modern Australian toilets use a different mechanism) sits at the bottom of the cistern and creates a watertight seal when the toilet isn't flushing. Over time, these seals harden, warp, or develop mineral deposits that prevent a proper seal. Water slowly leaks from the cistern into the bowl — often not enough to see clearly, but enough to trigger the inlet valve to keep topping up the cistern continuously.

Quick test: Put a few drops of food colouring into the cistern. Don't flush. Wait 10–15 minutes and check the toilet bowl. If colour appears in the bowl without flushing, your flush valve seal is leaking.

2. Faulty Inlet Valve

If you can hear water running into the cistern constantly — or if the cistern takes an unusually long time to fill — the inlet valve may be worn or failing. The inlet valve is responsible for stopping the water flow once the cistern reaches the correct level. When it fails, water keeps flowing in and either the cistern overflows into the bowl via the overflow tube, or the valve never fully closes.

Inlet valves can also have their float arm adjusted to control the water level in the cistern. If someone has knocked it or it's bent out of shape, the cistern may be set too high, causing it to constantly drain via the overflow.

3. Phantom Flushing ("Ghost Flushing")

If you've noticed your toilet suddenly refilling on its own without anyone flushing it — every 10, 20, or 30 minutes — this is what plumbers call phantom flushing. It's almost always caused by a slow leak from the cistern into the bowl via a faulty flush valve seal. The cistern quietly drains until it triggers the inlet valve to refill, which is what you hear. The cycle then repeats.

It's an easy thing to tune out, but it can waste a significant amount of water every day. If your toilet seems to flush itself, get it looked at.

4. A Damaged or Incorrectly Set Overflow Tube

Most cisterns have an overflow tube — a plastic tube that acts as a safety measure to direct excess water into the bowl rather than overflowing onto the floor. If the water level in the cistern is set too high (due to a maladjusted float), water will constantly trickle down the overflow tube into the bowl. You can usually see this by lifting the cistern lid and checking whether the water level sits above the top of the overflow tube.

What About Dual-Flush Systems?

Most Australian homes have dual-flush cisterns — the two-button system required by Australian Standard AS6400. These have their own specific inlet valves and flush seals that differ from older-style single-flush cisterns. The principles are the same, but the parts are different. A plumber who works on Australian toilets regularly will have the right components on hand.

Can You Fix It Yourself?

Some homeowners are comfortable replacing a flush valve seal or adjusting a float arm — the parts are available at most hardware stores and the job involves turning off the water at the cistern isolation valve and swapping the component. If you're handy and the cistern design is straightforward, it's doable.

That said, if you're not confident, or if you've replaced parts and the toilet is still running, it's worth calling a plumber. What looks like a seal issue can sometimes turn out to be a crack in the cistern, or a more complex valve failure. We can diagnose it in a few minutes and get it fixed properly.

How Much Is a Running Toilet Costing You?

Sydney Water charges around $2.45 per kilolitre for water usage in the first tier. A toilet leaking at 200 litres per day adds roughly 73,000 litres — or 73 kilolitres — per year. That's around $180 in wasted water per year from a slow leak. A significant leak can easily double or triple that figure. It's a repair that pays for itself quickly.