How Leak Detection Works: Thermal, Acoustic, and CCTV Methods Explained

A hidden water leak rarely announces itself. It seeps quietly behind a wall, under a slab, or beneath the yard, doing damage for weeks or months before you notice anything is wrong. By the time the signs are obvious, the leak has often been running long enough to inflate your water bill, stain your ceilings, or undermine the ground around a pipe. The good news is that finding a hidden leak no longer means smashing through walls or digging trenches across the lawn on a hunch. Modern detection equipment lets a plumber pinpoint the source first, then open up only the spot that needs fixing.

This guide explains the main signs of a hidden leak, why non-invasive detection saves you money, and how thermal imaging, acoustic detection, CCTV cameras, and tracer gas each work.

Signs you have a hidden leak

Most hidden leaks give themselves away through small, easily dismissed clues. Watch for:

• A rising water bill with no change in how much water you use. This is often the first and clearest sign.
• Damp patches on walls, ceilings, or floors that do not dry out, sometimes with bubbling paint or lifting tiles.
• Mould or a musty smell in a room, cupboard, or subfloor that was not there before.
• A warm spot on the floor, which can point to a leaking hot water line under a slab.
• The sound of running water when every tap and appliance is switched off.
• Reduced water pressure that has crept in gradually.

Any one of these on its own might be nothing. Two or three together usually mean water is escaping somewhere you cannot see. If you suspect a leak, call 0450 158 124 and we will connect you with a VBA-licensed plumber across south-east Melbourne who can locate it.

Why non-invasive detection matters

The old approach to a hidden leak was guesswork followed by demolition. A plumber would form a theory about where the water was escaping, open up a wall or break into a slab, and hope they had picked the right spot. If they were wrong, they patched it up and tried again somewhere else. Every wrong guess meant more damage, more reinstatement, and more cost passed on to you.

Non-invasive detection flips that around. The equipment finds the leak first, so the plumber knows exactly where to open up before they touch a wall or a floor. The maths is simple. A detection survey typically costs a few hundred dollars. Unnecessary excavation, re-rendering a wall, re-laying tiles, or re-pouring a section of slab can run into thousands, and that is before the actual repair. Paying for accurate detection almost always works out cheaper than paying tradespeople to chase a leak blind.

It also minimises disruption. Locating a slab leak with thermal imaging and acoustic gear means cutting one small access point instead of jackhammering across a whole room.

Thermal imaging: how it works

Thermal imaging uses a camera that sees heat rather than light. Every surface gives off infrared energy based on its temperature, and the camera renders that as a colour image. Where water is escaping, it changes the temperature of the surrounding material. A hot water leak warms the floor or wall around it. A cold water leak cools it. That temperature difference shows up clearly on the thermal image as a patch that does not match its surroundings.

Thermal imaging is well suited to finding:

• Slab leaks, where a pipe runs through or under a concrete floor.
• Underfloor heating leaks, where the warm line is easy to trace and any break stands out.
• Wall cavity leaks, where water tracking down inside a wall leaves a cool, damp signature.

The main limitation is that thermal imaging needs a temperature differential to work. If the leaking water is the same temperature as everything around it, there is nothing for the camera to see, which is why it is often paired with acoustic detection. It is also unreliable in direct sunlight, because the sun heats surfaces unevenly and masks the subtle differences the camera relies on. A plumber will usually work in shaded or indoor conditions to get a clean reading.

Acoustic detection: how it works

Acoustic detection listens for the leak. When water escapes from a pressurised pipe, it makes a distinct hissing or rushing sound as it forces its way out through the break. Specialist listening equipment, using ground microphones and amplified headphones, picks up that sound and makes it audible to the plumber. By moving the sensor across the floor or wall and listening for where the noise is loudest, the plumber can pinpoint the leak location, often to within a small area.

Acoustic detection is the go-to method for:

• Pressurised supply line leaks, both hot and cold, under slabs, in walls, and across the yard.

Its limitation is that it relies on pressure. The sound it detects is created by water being pushed out of a pressurised pipe. It is not useful for unpressurised drain leaks, because waste pipes carry water by gravity, not pressure, so there is no escaping hiss to listen for. For drain problems, a camera is the right tool.

CCTV drain cameras: how they work

A CCTV drain camera is a small waterproof camera on a flexible rod that the plumber feeds through the drain. It sends back live footage of the inside of the pipe, so instead of guessing what is wrong, you can see it. The camera shows the actual condition of the pipe along its length.

CCTV is the right choice for unpressurised drainage and sewer lines, and it reveals:

• Blockages, and what is causing them.
• Cracks and fractures in the pipe wall.
• Root intrusion, where tree roots have entered through a joint.
• General pipe condition, including sagging sections, scale, and material wear.

The simple rule is this. Use acoustic detection for pressurised supply lines, where the leak makes a sound but the pipe is sealed. Use CCTV for drains and sewers, where the pipe is open to a camera but carries no pressure for acoustic gear to detect. Many leak jobs use both, because a home has both kinds of pipework.

Tracer gas

For very small leaks, or leaks in awkward spots where acoustic and thermal methods cannot get a clear result, a plumber may use tracer gas. A safe, non-toxic gas mix is introduced into the empty pipe under pressure. Because the gas is lighter than air, it escapes through even a pinhole leak and rises to the surface, where a sensitive detector picks it up. It is a specialist method, but it can locate leaks that nothing else will find.

What happens after detection

Once the leak is located, the plumber will show you what they found. With CCTV that means playing back the footage of the pipe. With thermal imaging it means showing you the image with the leak highlighted. With acoustic or tracer gas they will mark the spot and explain how they confirmed it. You see the evidence, not just a verdict.

From there, the plumber explains the finding in plain terms, then quotes you for the repair before any cutting, digging, or fixing starts. Detection and repair are separate steps, and you decide how to proceed once you know what you are dealing with.

Leak detection costs in Melbourne (2026)

Price ranges based on real south-east Melbourne job data. Your plumber quotes after assessing the actual job. If something unexpected is uncovered during the work, they will pause, explain, and re-quote before continuing.

A combined survey, which pairs two or more methods, often gives the most reliable result on a leak that has resisted a single-method check, and it can work out better value than paying for separate visits.

South-east Melbourne context

Where you live shapes the kind of leak you are likely to have. Across the Casey and Cardinia growth corridor, slab-on-ground construction is the norm. Newer estates in Clyde North, Officer, and Pakenham are built with the pipework running through or under a concrete slab, which means hidden leaks tend to sit beneath the floor where thermal imaging and acoustic detection earn their keep. There is no subfloor to crawl into, so non-invasive location is the only sensible first step before anyone considers cutting the slab.

Closer to the bay, the picture is different. Many homes in Frankston and the Kingston area date from the copper pipe era of the 1960s through the 1990s. Copper supply lines from that period are prone to pinhole leaks as the metal ages and corrodes from the inside. These leaks are small, slow, and easy to miss until the water bill climbs or a damp patch appears, which makes them ideal candidates for acoustic detection and, where the pinhole is especially fine, tracer gas. If your home is from that era and your bill has crept up, a leak survey is worth doing before the damage spreads.

For more on what a survey involves, see the leak detection service page.

Need a leak found in south-east Melbourne?

VicPlumbers connects homeowners across south-east Melbourne with VBA-licensed plumbers experienced in thermal imaging, acoustic detection, and CCTV inspection. The plumber we connect you with locates the leak, shows you the evidence, and quotes the repair before any work begins.

Call 0450 158 124 or request a callback at vicplumbers.com.au.

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