Water leak in a wall: the signs, finding it, and the damage

A leak inside a wall is the kind you find late. The pipe is behind plasterboard or brick, the water runs down inside the cavity, and the first sign is often a symptom somewhere else: a stain low on the wall, paint that bubbles, a skirting board that swells, or a smell. By then the question is not only where the leak is, but what it has already cost — and, less obviously, whether being visible has cost you the water bill allowance.

The signs, and what each one points to

Water Corporation’s advice for spotting a hidden leak is to look for dampness or mould on walls, uneven paving and overgrown patches of lawn (Water Corporation, Expert advice on how to find and repair leaks at home). The national environmental health guidance on mould makes the same point about smell: some mould is not visible because it is behind a wall, and you may need to consult a professional if you can smell but can't see it (enHealth, Potential health effects of mould in the environment, p 3).

The last row of the table matters for diagnosis. A supply pipe is under pressure all the time, so it leaks whether or not anything is running, and the meter shows it. A waste pipe or a shower that leaks only when used will not move the meter at all. That one distinction can tell you whether to call a leak detection plumber or a waterproofer.

Confirming it: the meter test, then a plumber

Turn off every tap and every water-using appliance, read the meter, and wait. The utilities give different times: South East Water says wait for 20 minutes, Water Corporation 15 minutes on the red numbers, Sydney Water at least an hour (South East Water, Leaks). If the numbers move, there is a leak on the supply side. Then turn off the isolation valves you can find — toilets, hot water unit, under sinks — one at a time; if the meter stops, the leak is on the branch you just closed.

Finding it inside a wall is a job for a licensed plumber, and in Western Australia Water Corporation reminds owners that all plumbing work must be done by one. A leak detection plumber uses equipment to narrow down the spot before anything is cut; Icon Water’s allowance form asks the plumber to state how the leak was detected. hipages publishes leak detection at $168 to $400 nationally, indicative only. Ask for the plumber’s findings in writing: where, what failed, how it was found and where the water went. The same statement serves the water utility, the insurer and, in a unit, the owners corporation.

Why a visible leak can cost you the water bill allowance

Each water utility covered on this site has a scheme that refunds part of the charges for a hidden leak. The catch for a leak in a wall is the word hidden. Several schemes are written so that a leak that has started showing inside the house falls outside them:

The practical lesson runs backwards from the damage: the leak most likely to be refunded is the one the meter catches before the wall does. Reading the meter at the same time every month, or checking the bill against the same quarter last year, is how that happens. Once a leak is visible, fix it straight away anyway; the allowance is a refund of water charges, and the wall costs more. The leak detection guide sets out every utility’s scheme, and the city pages for Sydney, Brisbane, Perth and Canberra go through the conditions.

The damage: drying the cavity, and when the wall comes open

Water in a wall cavity does not dry the way a wet floor does. The plasterboard is wet from the back, the bottom plate and insulation hold water, and the cavity has little air movement. A restorer may dry it in place, with air directed into the cavity through small holes and readings taken daily, or may cut out the bottom of the sheet to open it up. NSW Health’s rule for what stays applies: discard material that cannot be cleaned and dried completely (NSW Health, Mould factsheet).

On published prices, drying two rooms over five days is $2,269 excluding GST on one restorer’s rate card, and replacing plasterboard is $15 to $20 per square metre supplied and installed. Where the wall backs onto a shower, the NCC’s wet area requirement is that water is prevented from penetrating behind fittings and linings and into concealed spaces (National Construction Code 2022, Volume Two, H4P1 (Wet areas)); a leak there is a waterproofing failure, and the leaking shower guide covers the building warranty. Mould on walls covers telling a leak apart from condensation or rising damp.

Insurance, strata and rentals

Insurance. Standard home cover names bursting, leaking, discharging or overflowing of fixed apparatus, fixed tanks or fixed pipes used to hold or carry liquid of any kind (Insurance Contracts Regulations 2017 (Cth) reg 19(1)(a)(vi)). That is the water damage. The pipe or fitting that let go is a separate question, and wear and tear, rust or corrosion is taken back out of standard cover (Insurance Contracts Regulations 2017 (Cth) reg 19(2)(b)). A leak that ran for months inside a wall is exactly where an insurer will look at that exclusion, which is why the plumber’s written cause and the failed section of pipe are worth keeping. Plumbing cover goes through it.

Units. A pipe in a shared wall may serve more than one lot. In NSW the owners corporation must properly maintain and keep in a state of good and serviceable repair the common property (Strata Schemes Management Act 2015 (NSW) s 106(1)), so a leak in a common pipe is its repair; a pipe serving only your lot may be yours. The strata plan and the by-laws decide which; report it in writing to the strata manager either way.

Rentals. A burst water service is an urgent repair in NSW, where a tenant who arranges it can be repaid up to $1,000 (NSW Government, Getting repairs done; Residential Tenancies Act 2010 (NSW) s 64(3)), and in Queensland emergency repairs include a burst water service or a serious water service leak (Residential Tenancies and Rooming Accommodation Act 2008 (Qld) s 214). Tell the landlord or agent in writing the day you notice it.

Common questions