You flush, walk away, and ten minutes later the cistern is still hissing. So you jiggle the button, maybe press it again, and it stops for a while. Then it starts up at 2 am. Sound familiar? A toilet keeps running because water is escaping somewhere it should not be, and a plumber in Sydney will find the cause by working through four checks in a specific order rather than guessing at parts.
Most of the advice you will find online is written for American single-flapper toilets, which is not what sits in the majority of Australian bathrooms. The steps below are based on the dual flush cisterns our team actually opens up every week across Sydney.
Why Homeowners Replace the Wrong Part
Search “toilet keeps running” on any forum and you will find the same advice: replace the flapper. The problem is that most Australian dual flush toilets do not use a traditional flapper. They use a flush valve seal (sometimes called a toilet washer or outlet valve seal) that sits at the bottom of the cistern, and it works differently.
Homeowners buy the wrong part, install it, and the toilet keeps running. Or they replace a seal that was not the issue, while the actual cause (a maladjusted fill valve or worn inlet washer) goes untouched. A plumber avoids this by following a diagnostic sequence rather than guessing.
Check 1: The Water Level and Overflow
The first thing a plumber looks at is where the water sits in the cistern relative to the overflow tube. If the water level is above the overflow, the cistern is overfilling and water is constantly trickling into the bowl. The running sound you hear is the fill valve working overtime to replace water that is draining away through the overflow.
What the plumber does: Lifts the cistern lid, checks whether water is flowing into the overflow tube, and adjusts the float or fill valve height so the water stops approximately 10 to 15 mm below the top of the overflow. Then flushes and watches to confirm the water stops at the correct level.
This is often the entire fix. A float arm that has slipped or a fill valve bumped during cleaning can shift the water level just enough to cause a constant trickle.
Check 2: The Flush Valve Seal and Seat
If the water level is correct and the toilet is still running, the next check is the flush valve seal at the base of the cistern. In a dual flush system, this seal creates the watertight barrier between the cistern and the bowl. When it wears out, hardens, or collects grit, water leaks past it into the bowl.
The food colouring test: A simple way to confirm this at home is to add a few drops of food colouring to the cistern water, then wait 15 minutes without flushing. If the colour appears in the bowl, water is getting past the flush valve seal.
The plumber will remove the flush valve assembly, inspect the rubber seal for cracks or hardening, and check the valve seat for corrosion or mineral buildup. In areas with hard water (parts of Western Sydney certainly qualify), calcium deposits on the seat can prevent the seal from closing properly even when the seal itself looks fine.
Check 3: The Fill Valve and Inlet Washer
If the flush valve seal and seat are fine, the fill valve is next. The fill valve controls the flow of water into the cistern after each flush. When the inlet washer inside it wears out or the valve body develops a fault, it can fail to shut off completely or cycle on and off intermittently (sometimes called “ghost flushing”).
Signs the fill valve is the issue:
- The toilet runs for a few seconds, stops, then starts again on its own.
- You hear a hissing sound from the cistern even when nobody has flushed.
- The cistern takes noticeably longer to refill than it used to.
Replacing a fill valve is a straightforward job for a licensed plumber and typically resolves the problem in a single visit. It is also one of the most common general plumbing services calls our team handles across Sydney.
Check 4: The Less Obvious Causes
Most running toilets are resolved by checks one through three. But occasionally the cause is something less common.
- Corroded valve seat. If the brass or plastic seat that the flush valve seal presses against is pitted or rough, no new seal will sit flat enough to stop the leak. The seat itself needs to be replaced or the entire flush valve assembly swapped out.
- Faulty button assembly. On dual flush toilets, the push buttons connect to the flush valve via a cable or linkage. If the linkage is misaligned or the button is sticking, the valve may not close fully after a flush.
- Hard water buildup inside the cistern. Mineral scale can accumulate on internal components over time, restricting movement and preventing seals from seating properly. A thorough clean and descale can resolve the issue without replacing any parts.
If you notice water pooling around the base of the toilet rather than just hearing it run, that is a different problem entirely and may indicate a failed pan seal or a cracked cistern. In that case, contact an emergency plumber Sydney before the leak causes damage to your flooring or the ceiling below.
What Is DIY and What Needs a Plumber
- DIY-friendly: Adjusting the float height, running the food colouring test, cleaning visible grit from the flush valve seal, and checking whether the flush button is sticking.
- Call a plumber: Replacing the fill valve or flush valve assembly, dealing with a corroded seat, diagnosing ghost flushing, and anything involving the water supply connection or the pan seal.
The general rule is: if the fix involves swapping a single component you can buy at Bunnings, it is reasonable to try it yourself. If the toilet is still running after your first attempt, a licensed plumber will diagnose it faster and avoid the trial-and-error cycle that ends up costing more in parts than a single service call.
For similar guidance on another common household repair, have a look at our guide on how to fix a dripping tap.
FAQs and Tips
How much water does a running toilet waste?
According to the NSW Government, a continuously running toilet can waste between 60,000 and 96,000 litres of water per year (source: RWCC NSW, 2026). Even a slow, intermittent leak adds up quickly, and it will show on your water bill before you notice it anywhere else.
Why does my toilet run for a few seconds then stop on its own?
This is usually the fill valve topping up a small amount of water that has leaked past the flush valve seal. The cistern loses water slowly, the float drops, and the fill valve kicks in briefly to refill. It is often called ghost flushing. The food colouring test will confirm whether this is happening.
Is a running toilet an emergency?
Not typically, but it should not be left for weeks. The water waste is significant and the constant flow can mask a more serious issue. If water is leaking onto the floor or through to a lower level, treat it as urgent and call a plumber immediately.
Can hard water cause a toilet to keep running?
Yes. Hard water deposits can build up on the flush valve seat, preventing the seal from closing properly. A descale and clean of the internal components will often fix the problem without replacing any parts.
My toilet is also slow to drain. Could the two problems be related?
A slow-draining toilet is usually a blockage issue rather than a cistern problem. If your toilet is running and slow to drain, you may have two separate issues. Our guide on how to unblock a toilet covers the drainage side in detail.
Sigma Plumbing is a licensed, Sydney-based plumbing company serving homeowners, businesses, and strata properties across Western Sydney and Greater Sydney. If your toilet keeps running and you have already tried the basics, call us on 0426 582 778 or visit sigmaplumbing.com.au to book a service call.
