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Why Does My Toilet Keep Running? Common Causes and DIY Fixes
Why Does My Toilet Keep Running? Common Causes and DIY Fixes — a Prime Plumbing & Gasfitting plain-English guide to toilet keeps running for Melbourne homeowners. Below we cover what works, what doesn't, and when to call a licensed plumber.
Quick answer: A constantly-running toilet is almost always one of three things: a worn-out flapper (the rubber valve at the bottom of the cistern that seals after each flush), a faulty fill valve (the assembly that refills the cistern), or a misadjusted float (which tells the fill valve when to stop). Most fixes take 15–30 minutes and cost under $25 in parts. The water waste from a constantly-running toilet adds up fast — typically 25,000–80,000 litres per year and $200–$650 on your water bill. If you're not comfortable opening the cistern, or if you've replaced the flapper and fill valve and the toilet still runs, the problem is usually a hairline crack in the overflow tube or flush valve seat — that's a plumber call.
How a toilet actually works (quick refresher)
Once you understand the cycle, diagnosis is easy. Lift the cistern lid:
- You press the flush button — the flapper or flush valve at the bottom of the cistern lifts, and water rushes down into the bowl
- Water in the cistern empties through the flush valve seat into the bowl
- Flapper or flush valve drops back down as the water level drops, sealing the cistern from the bowl
- Float drops with the water level, which signals the fill valve to open
- Fill valve refills the cistern through the inlet hose, and partially refills the bowl through the small bowl-fill tube that goes into the overflow standpipe
- Float rises with the water, and at the set level, signals the fill valve to shut off
- Cistern is now ready for the next flush
A toilet "keeps running" when one of these steps gets stuck — water keeps flowing because the cycle never properly completes.
The three common causes
1. Worn-out flapper (most common — 60% of running toilets)
What it is: the rubber or silicone disc at the bottom of the cistern that seals the flush valve seat. After hundreds of thousands of flushes (usually 5–10 years), the rubber hardens, warps, or develops mineral build-up — and stops sealing properly. Water seeps from cistern → bowl, the cistern level drops, and the fill valve keeps cycling on to top it back up.
How to diagnose:
- Take the cistern lid off (lift straight up — most are just resting in place)
- Add a few drops of food colouring to the cistern water
- Wait 15 minutes without flushing
- If the dye appears in the bowl, your flapper is leaking
How to fix:
- Turn off the isolation valve below the cistern (rotate clockwise until it stops — should go quarter-turn or full-turn depending on style)
- Flush to empty the cistern
- The flapper is usually attached by two ear-mounts on the flush valve and a chain from the flush lever. Unhook the chain, slide the flapper off the ears
- Buy a replacement at Bunnings, Reece, or your hardware store ($8–$22). Bring the old one for matching, or buy a "universal" flapper which fits 90% of toilets
- Slide the new one on, reattach the chain (the chain should have about 12mm of slack when the flapper is closed)
- Turn isolation valve back on, let cistern fill, test flush
This is the most common fix and the most DIY-friendly.
2. Faulty fill valve (about 25% of cases)
What it is: the tall assembly on the left side of the cistern that controls the inflow of water. Older toilets use a brass ballcock with a horizontal arm and metal float ball; newer ones use a vertical-action plastic fill valve (Fluidmaster-style). They wear out — internal washers fail, mineral build-up jams the diaphragm, or the shut-off mechanism stops responding to the float.
How to diagnose:
- Listen — a fill valve that's faulty usually makes a continuous low hiss, sometimes intermittent ticking, even when no one's flushed recently
- Lift the float manually with the cistern open. If lifting the float stops the water, the valve is fine and your problem is elsewhere (flapper or float adjustment). If lifting the float doesn't stop the water, the fill valve is faulty
- A trickle of water entering the cistern from the inlet when no one flushed = fill valve issue
How to fix:
- Turn off the isolation valve and flush to empty
- Disconnect the water supply hose at the bottom of the fill valve (it's a plastic or brass nut — finger-tight in most cases, otherwise gentle adjustable wrench)
- Unscrew the locking nut on the underside of the cistern
- Lift the old fill valve out
- Install replacement (Fluidmaster Pro 400, Caroma equivalent, etc — $18–$45) — these have height adjusters so you can match your existing cistern depth
- Reconnect water supply, restore isolation valve, adjust float so the water level sits about 25mm below the top of the overflow tube
3. Misadjusted float (about 10% of cases)
What it is: the float (ball or cup) that rides on the water surface. It signals the fill valve when the water reaches the desired level. If it's set too high, the water rises above the overflow tube — and water continuously overflows down into the bowl. From outside, this looks identical to a leaking flapper.
How to diagnose:
- Look at the water level in the cistern. The water should sit about 25mm below the top of the overflow tube.
- If the water is at or above the overflow tube top, water is going down it constantly — this is your problem.
- If you can see water trickling into the overflow tube while the cistern is at rest, that's the giveaway.
How to fix:
- Most modern fill valves have a clip or screw on the float assembly that adjusts the cut-off level. Lower the float by about 12–25mm.
- Brass ballcock styles: gently bend the metal float arm downward (it's designed to be bent for adjustment) until the water level drops below the overflow.
- Test by flushing and watching the refill cycle.
Less common causes (when the basics don't fix it)
If you've replaced the flapper, fill valve, and adjusted the float and it's still running, you've crossed into licensed plumber territory. Common deeper issues:
- Cracked overflow standpipe: water's running down a hairline crack invisible from outside. Usually requires replacing the entire flush valve assembly — $150–$300 plumber call.
- Cracked flush valve seat: same scenario but the crack is in the seat the flapper closes against. Same fix range.
- Pinhole in cistern bottom: rare but happens on older porcelain. Often means cistern replacement.
- Concealed cistern in modern wall-hung toilets: in-wall cisterns can have failed inlet valves that are tricky to access. Plumber call.
- Overpressure on the inlet line: if mains pressure is above 500 kPa, fill valves wear out fast and may not seat properly. A pressure-limiting valve on the household inlet ($120–$280 fitted) extends fill valve life.
When to call a plumber
DIY-safe scope: replacing flappers, fill valves, adjusting floats, and tightening hose connections.
Call a licensed plumber when:
Plumber callouts for a running toilet are typically $150–$300 including parts, depending on what's wrong.
Frequently asked questions
Bring the old one with you to Bunnings or your hardware store. There are about a dozen common types — round 50mm, oblong, tank-style, dual-flush styles for Caroma — and they don't interchange perfectly. Or buy a "universal" flapper which fits roughly 80% of toilets and includes adapters.
That's a slow flapper leak. The flapper isn't fully sealing, water gradually seeps from cistern to bowl, the level drops to the float's cut-on point, the fill valve runs briefly to top up. Same fix — replace the flapper.
Almost never. Cisterns are essentially porcelain boxes that don't wear out. Their internals (flapper, fill valve, flush mechanism) wear out and are inexpensive to replace. Cistern replacement is justified if there's a crack in the porcelain itself or if you're modernising for water-efficiency reasons (older 9L cisterns vs modern 4.5/3L dual-flush).
Slow flapper leak. Water gradually drains from cistern to bowl, hits the fill valve's cut-on level, and a refill cycle starts. Replace the flapper.
That's the bowl-refill cycle filling water back into the bowl through the small tube that goes into the overflow standpipe. Normal for most toilets. If it runs longer than 90 seconds, the fill valve refill ratio may need adjusting (most have an adjuster screw on top).
No — every modern toilet has its own isolation valve where the water supply meets the cistern (usually behind or below the toilet, near the floor). Turn that quarter or half-turn until it stops. Test by flushing — if the cistern doesn't refill, the isolation is working.
If the isolation valve is seized, painted over, or behind the cistern in a wall-hung install, you'll need to turn off the water at your property's main shutoff (usually at the front of the house near the meter). After the repair, ask your plumber to install or replace the isolation valve so future repairs are easier.
Three checks: (1) is the chain too short, holding the flapper slightly open? (2) is the new flapper seated correctly on the flush valve seat? (3) is there mineral build-up or roughness on the flush valve seat that's preventing a proper seal? If all three check out, your problem is the fill valve or float — work through those next.
How much water is a running toilet costing you?
Real numbers from Yarra Valley Water and South East Water utility data:
- Slow leak (cistern slowly seeping into bowl): 20–80 litres per day, ~$50–$200 per year
- Steady running (continuous trickle): 200–500 litres per day, ~$200–$650 per year
- Major running (constant flow at audible rate): 1,000+ litres per day, $1,000+ per year and rising
A $20 flapper saves hundreds. There's no toilet repair where DIY is more cost-effective.
- When did the issue start?
- Is it isolated to one fixture or multiple areas?
- Are there any visible leaks, smells or unusual sounds?
- Have you turned off the relevant isolation valve?
For toilet repairs that go beyond DIY scope, see our Toilet Repairs Melbourne page. Leaking taps and other bathroom plumbing is covered on our Leaking Tap Repairs Melbourne page. If a running toilet is making your water bill jump and you want a property-wide plumbing inspection, contact us.
