Fix a Running Toilet in Under an Hour (Replace the Fill Valve & Flapper)

✍️ Peter Kim, EIJ Construction📅 June 10, 2026⏱ 7 min read

A running toilet can waste up to 200,000 litres of water a year — and add hundreds of dollars to your water bill. In most cases the fix costs $20–$40 in parts and takes under an hour with no special skills. Here's how to diagnose and fix the two most common causes.

Diagnose First: Which Part is Failing?

Put a few drops of food colouring in the cistern (tank). Wait 15 minutes without flushing. If colour appears in the bowl, your flapper is leaking. If the water level is up at the overflow pipe, the fill valve (ballcock) isn't shutting off properly. If both tests fail, replace both — they're cheap and the job is already half done.

Tools & Materials

Adjustable wrench, sponge, bucket, replacement fill valve (Caroma and Geberit are reliable Australian brands, $15–$40), replacement flapper (match the brand on your cistern, $5–$15), and rubber gloves. Most hardware stores stock universal kits. Take a photo of your cistern before going shopping.

Step 1: Turn Off the Water Supply

Find the isolation valve on the pipe behind or below the toilet and turn it clockwise until it stops. Flush the toilet to empty the cistern. Use a sponge to mop out any remaining water in the bottom of the cistern. Lay a towel on the floor — there will be drips.

Step 2: Replace the Flapper

Unhook the chain from the flush lever. Unhook the old flapper from the pegs on either side of the flush valve seat. Clip the new flapper onto the same pegs and reconnect the chain, leaving about 1–2cm of slack. Too much slack and the flapper won't seal; too tight and it won't drop back down after flushing.

Step 3: Replace the Fill Valve

Disconnect the water supply line from the bottom of the cistern (have your bucket ready — some water will come out). Remove the locknut holding the old fill valve in place. Lift out the old valve. Drop in the new fill valve, adjust the height per the instructions, and hand-tighten the locknut. Reconnect the water supply line.

Step 4: Test and Adjust

Turn the water supply back on slowly and let the cistern fill. Adjust the water level on the new fill valve so it sits about 25mm below the top of the overflow pipe — most valves have a simple clip or screw adjustment. Flush a few times and check for leaks at the supply line connection. Done.

Queensland Note: Plumbing Licence Requirements

In Queensland, homeowners can perform minor maintenance like replacing a toilet cistern filling valve, flapper, or flush button on their own home — this is classified as 'owner-occupier work'. However, moving or installing new plumbing fixtures, altering waste pipes, or any work on gas connections must be done by a licensed Queensland plumber. When in doubt, check with the QBCC.

💡 Expert Tips from EIJ Construction

→Take a photo of your toilet model number (inside the cistern lid) before buying parts — compatibility matters
→Replace both flapper and fill valve at the same time — the cost difference is minimal if you're already in there
→After reassembly, add a few drops of food colouring in the cistern again to confirm the flapper is sealing
→If the cistern still runs after replacing both parts, the flush valve seat may be corroded — call a plumber
→Caroma and Geberit parts are widely available in QLD and have good support — avoid unknown imports

👷

Written by Peter Kim

QBCC-licensed waterproofing specialist & founder of EIJ Construction. Gold Coast · Sunshine Coast · SEQ.

Get a Free Quote