Quick Answer
To detect a hidden leak, do the water meter test: shut off all water, note the meter, and re-check it two hours later. If it moved, you have a leak. Back that up by watching for a spiking water bill, damp patches, musty smells, low pressure, and the sound of running water when everything is off.
The worst plumbing leaks are the ones you never see. A slab leak under the floor, a weeping joint inside a wall, a cracked pipe under the garden — they run for weeks or months, rotting timber and swelling plaster, before a single visible drop ever appears.
By the time water stains the ceiling, the damage bill is often ten times what the original repair would have cost. Here’s how to catch a hidden leak early.
6 warning signs of a hidden leak
1. Your water bill jumped for no reason. A sudden increase with no change in usage is the single most reliable red flag.
2. A musty or damp smell. Persistent mustiness, especially in one room, points to moisture trapped behind a wall or under a floor.
3. Stains or bubbling paint. Yellow-brown marks on ceilings and walls, or paint that blisters and peels, mean water is getting in.
4. A drop in water pressure. If a pipe is leaking, less pressure reaches your taps and shower.
5. The sound of running water. A faint hiss or trickle when every tap is off is a classic sign of a pressurised leak.
6. An unusually green or soggy patch of lawn. An underground supply leak feeds one area of garden far better than the rest.
The water meter test (do this first)
This is the most reliable DIY check there is, and it takes two minutes of active effort:
Step 1. Turn off every tap inside and outside, and make sure no appliance is using water — no dishwasher, washing machine, or ice maker running.
Step 2. Find your water meter (usually at the front boundary) and write down the exact reading, including the small dials.
Step 3. Don’t use any water for two hours, then read the meter again.
If the reading has changed at all, water is escaping somewhere between the meter and your taps — you have a leak. Many modern meters also have a small spinning leak indicator; if it’s turning while all water is off, that confirms it.
Where hidden leaks like to hide
Some spots account for the majority of the concealed leaks we find in Sydney homes:
Under the slab. Supply pipes running beneath a concrete floor. Warm patches on the floor can indicate a hot water slab leak.
Inside walls. Old soldered joints and corroded pipework behind kitchens and bathrooms.
Under the house. In homes with a subfloor, dripping joints often go unnoticed for years.
In the garden. The main supply line from the meter to the house, especially where old galvanised or poly pipe has been in the ground for decades.
The toilet cistern. A silent internal leak here can waste thousands of litres — see our guide on why your toilet keeps running.
DIY checks you can do today
Beyond the meter test, walk the house and check under every sink, behind the toilet, around the dishwasher and washing machine, and at the base of your hot water unit. Look for water stains, corrosion, swollen cabinetry, and mould. Feel for damp spots on floors and walls. Reading your bill closely also helps — our Sydney water bill guide shows how to compare usage across billing periods.
When to call a plumber
If the meter test confirms a leak but you can’t see where it is, that’s the moment to call. A licensed plumber uses acoustic listening gear, thermal cameras, and pressure testing to pinpoint the leak to within centimetres — no guesswork, no tearing up floors on a hunch. Finding it precisely means we open up only the small area that actually needs repairing, which keeps both the disruption and the bill down.
FAQs
How much water can a hidden leak waste?
Even a small pinhole leak can waste hundreds of litres a day. Left running for a full quarter, that easily becomes tens of thousands of litres — and a water bill that’s hundreds of dollars higher than usual.
Does home insurance cover hidden leak damage?
Many policies cover sudden water damage but exclude damage from a leak that’s been slowly happening over time. That’s exactly why early detection matters — the sooner it’s found and fixed, the stronger your position and the smaller the damage.
Is leak detection expensive?
Professional leak detection is a fraction of the cost of the water damage a hidden leak causes if left running, and far cheaper than exploratory digging. We give you an upfront price before we start, with $0 call-out fee.
Think You Have a Hidden Leak?
Mr. Clog uses acoustic and thermal leak detection to find hidden leaks fast across Sydney — no unnecessary digging, no guesswork.