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Underground Water Leak Signs: How to Find a Hidden Leak in Your Yard
Underground Water Leak Signs: How to Find a Hidden Leak in Your Yard, a Prime Plumbing & Gasfitting plain-English guide to underground water leak signs for Melbourne homeowners. Below we cover what works, what doesn't, and when to call a licensed plumber.
Quick answer: An underground water leak shows itself in five main ways: an unexplained spike in your water bill (the most reliable sign, a quiet leak adds litres to every bill), wet or unusually green patches on the lawn or driveway in dry weather, reduced water pressure at fixtures inside the house, the sound of running water when no taps are open, and mud, mould or surface settlement above the pipe route. The water meter test confirms it: turn off all fixtures, watch the meter for 15 minutes, if it's still moving, water is escaping somewhere. Underground leaks rarely fix themselves and usually get worse; locating the leak early with acoustic detection is far cheaper than waiting for visible damage.
Why underground leaks matter more than visible ones
A visible leak, a dripping tap, a leaking pipe under the sink, is annoying. Water pools, you mop it up, you call a plumber, it gets fixed. The damage is contained and you can see what's happening.
An underground water leak is different. The pipe is typically buried 300-600mm below ground, the leak is invisible from the surface for weeks or months, and the water silently saturates the soil around the pipe and can:
- Wash away soil bedding that supported the pipe, accelerating the failure
- Erode below paths, driveways and slabs, leading to settlement and cracked concrete
- Saturate footings, undermining foundation walls in extreme cases
- Run thousands of litres per week, at metered cost
- Slowly raise damp under the house, in cavity walls, or under tile floors
By the time you see surface symptoms, you've often already paid for hundreds of dollars of wasted water and the leak's been damaging structures for some time.
The single most useful habit: watch your water bill quarter-by-quarter and follow up on unexplained increases.
The five main signs
1. The water bill has gone up for no reason
The most reliable indicator. If your usage has been stable for years and a bill arrives that's 30%, 50%, 100% higher than normal, and you haven't done anything different (no garden watering, no new occupants, no pool filling), assume a leak until proven otherwise.
A small underground leak (say a slow weep at a buried fitting) can run 30-100 litres per hour continuously. Over a 3-month billing period, that's between 60,000 and 200,000 litres. Even at residential water rates, that's a meaningful bill spike.
Compare your bill to:
- The same quarter last year
- The previous quarter
- Any seasonal patterns you'd normally expect (summer garden use, winter low use)
If your latest bill doesn't fit the pattern, run the meter test (below) before assuming it's just usage variation.
2. Wet or unusually green patches on the lawn or driveway
A leaking pipe under the lawn produces a localised area of saturated soil above the leak. Two visible signs:
In dry weather: a patch that stays wet or muddy when the rest of the lawn is dry. Grass is greener and lusher in the patch than elsewhere because of the constant water + nutrient supply (water mains pipes are pressurised mains-fed; the leaking water is potable supply, not drain water).
Under driveways or paths: water seeping out at the edge of paving, or soft "spongy" sections where the soil under the slab has been washed away.
These signs are diagnostic but lag the leak by weeks or months: by the time you can see surface saturation, the leak's been running long enough to fully wet the surrounding soil.
3. Reduced water pressure at fixtures
A significant leak between the meter and your house drops pressure available to indoor fixtures. You'll notice:
- Showers feel weaker than they used to
- Multiple fixtures running at once produce noticeably less flow each
- Garden sprinklers struggle
- Filling a sink takes longer than it used to
Pressure changes can have other causes (a partially-closed valve, sediment in fixtures, an old PRV at the meter), but combined with other signs, low pressure points at a leak.
4. The sound of running water when nothing's on
In a quiet house at night, walk past the area where you suspect the supply pipe runs (typically along one side of the house, between the meter and the entry point). Sometimes you'll hear a faint hissing or running-water sound: the leak itself, or water flowing through pipework that "shouldn't be drawing".
This is a useful sign in single-storey homes where you can stand close to the buried pipe. It's less reliable in upstairs floors, where pipework is internal.
5. Surface signs above the pipe route
The supply pipe between the water meter at the boundary and the house typically runs in a straight line. Look for:
- Soil settlement along the route, slight depression in lawn or paving
- Mud or wet soil appearing where it shouldn't
- Mould growth on the wall closest to where the pipe enters the house
- Damp patches at the slab edge inside the home, particularly during dry weather (when external rainwater isn't a confounding factor)
These are usually late-stage symptoms, by the time the surface shows, the leak has been running long enough to have done meaningful work.
The 15-minute meter test
This test confirms whether you have a leak somewhere on your property. It takes 15 minutes and costs nothing.
- Turn off every fixture in the house: taps, washing machine, dishwasher, toilet (don't flush), garden sprinklers, pool autofill if you have one. Make sure no one will use water during the test.
- Find your water meter: usually at the front of the property near the boundary. Lift the cover.
- Read the meter: note the reading exactly, particularly the small "leak indicator" wheel on most modern meters (it's a tiny rotating disc or red triangle that moves when even small flow is happening).
- Wait 15 minutes without using any water.
- Read the meter again.
If the reading has changed, or the leak indicator has moved, you have a leak somewhere on your property. The leak rate over 15 minutes gives you a rough idea of severity:
- Leak indicator moves slowly but the meter reading hasn't visibly changed small leak (drip, weep)
- Leak indicator moves visibly and meter reading shows a small change moderate leak
- Meter reading changes significantly in 15 minutes significant leak; act quickly
If the meter is still after 15 minutes, no leak exists at your fixtures, but you may still have one upstream of the meter (rare but possible, that's the water authority's responsibility).
Where underground leaks normally are
Most residential underground leaks we attend are at one of these locations:
- Joints in the buried supply pipe between the meter and the house, particularly threaded fittings or compression couplings buried decades ago
- Old galvanised supply pipework that's been corroding internally for 50+ years and finally pinholed through
- PVC fittings damaged during landscaping: buried tree roots, fence posts driven through pipes, vehicle traffic compacting over the pipe
- Polypipe at compression-joint fittings: these can fail if not properly tightened or if ground movement has stressed the joint
- Old copper supply lines: particularly where corrosive soils have eaten through the pipe wall
Service area
Prime Plumbing & Gasfitting locates underground leaks across Melbourne's eastern, south-eastern, inner-east and bayside suburbs. See all suburbs we service
Book leak detection
Call 0475 407 670 or send through the contact form. Tell us what you've noticed (bill increase, wet patch, low pressure) and how long you've seen the symptoms, it helps us bring the right detection gear.
- BPC #103414: Plumbing Industry Commission licensed
- 4.8 stars on Google
- Acoustic detection equipment carried on every callout
- Fixed-price written quote before any work
How a plumber finds the leak
Locating the leak without digging up the whole yard requires specialised equipment.
Acoustic leak detection
The most common method. A sensitive microphone is pressed against the ground at intervals along the pipe route. Pressurised water escaping from a leak produces a characteristic high-frequency hiss. The plumber listens for the sound, marks where it's loudest, and that's where to dig.
Modern detection equipment can commonly locate a leak to within 1-2 metres on a well-buried pipe.
Pressure mapping
By isolating sections of the supply line and measuring pressure at different points, the plumber can narrow down which section of pipe is bleeding pressure. Useful when the leak is small or in an area where surface noise interferes with acoustic detection.
Tracer gas
For deep or quiet leaks, a non-toxic gas (often nitrogen-hydrogen mix) is injected into the line. The gas escapes through the leak and rises to the surface, where a detector picks it up. More involved than acoustic but very accurate for hard-to-find leaks.
Thermal imaging
A thermal camera shows temperature differences on the surface, wet ground from a leak is cooler than surrounding dry ground, especially after sunshine. Useful as a confirming check rather than a primary location method.
For underground leak detection in Melbourne, our burst pipes service covers acoustic detection, repair excavation, and slab leak work.
What to do once you suspect a leak
- Run the meter test to confirm something is leaking
- Check obvious indoor fixtures: toilet running silently, dripping tap somewhere, washing machine valve weeping. Sometimes "underground" leaks turn out to be a slow toilet fill valve that runs 24/7.
- Check all visible plumbing: under sinks, behind washing machines, at the hot water unit, at the meter
- Walk the supply route outside, looking for surface signs
- Call a plumber if confirmed leak and not obvious indoor source
Don't dig speculatively unless you have strong surface evidence and you're prepared to call a plumber after locating the leak. Acoustic detection costs less than excavating in the wrong place.
When to call a licensed plumber
The Building and Plumbing Commission maintains a plumbing licence search so you can verify your plumber.
Frequently asked questions
The leak repair itself normally isn't covered (classed as maintenance), but water damage caused by the leak (settled slab, damaged wall, ruined garden) can be covered by some home insurance policies. Some insurers offer underground services add-on cover that includes water and sewer line repairs. Worth checking your specific policy or asking your insurer about cover for "hidden water damage" before assuming it isn't covered.
Not really. Even if the leak rate is small, it's gradually doing damage to whatever the water is saturating, soil bedding, paving, footings, slab. A leak that's costing $50/quarter in water can be doing $500-$5,000/year of cumulative structural damage that's invisible until it isn't. Plus the water bill increases over time as the leak grows.
A small leak running at 30 L/hour adds up to about 263,000 litres per year, at residential metered rates in Melbourne, roughly $700-$1,000/year in water alone. A larger leak at 100 L/hour is well over $2,500/year. Plus the structural damage costs.
The supply pipe between the meter and your house is your responsibility. The supply pipe upstream of the meter is the water authority's. If the leak is downstream of the meter, you pay; if it's upstream, the authority's responsibility. Most underground leaks at residential properties are downstream.
Over time, yes. Saturated soil under a slab can heave or settle, depending on soil type, clay soils swell when wet and shrink when dry, creating differential movement. A small leak running for years can produce visible cracking in slab and walls. This is why underground leak detection should happen sooner rather than later.
Sometimes. A single pinhole in 50-year-old copper often signals that the rest of the pipe is similarly degraded, repairing one spot may just shift the problem. The economical decision is sometimes a partial replacement (renew the most-degraded section) or full supply pipe replacement (renew from meter to house in modern pipe). We'll quote both at diagnosis.
Maybe. If you have a clear, single-route supply line with accessible isolation points, you can isolate sections and watch the meter to narrow down which section leaks. But most residential supply runs aren't that clean, and acoustic detection typically saves time vs DIY pressure testing. Worth a try if you're handy and patient; otherwise, book a plumber with the right gear.
How much does it cost?
Indicative ranges only, depends on access, depth, leak location, and whether the repair involves excavation through paving:
- Acoustic leak detection callout: typically $300-$550
- Combined detection + minor repair (visible pipe): typically $550-$1,000
- Excavation + repair under lawn: typically $1,000-$2,500
- Excavation + repair under driveway/paving: typically $2,000-$5,000+ (the paving repair often costs more than the plumbing)
- Full supply pipe replacement (meter to house): typically $3,500-$8,000+
We always quote in writing first.
- 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?
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