Sewage Smell in Your Yard? Cracked Sewer, Overflow Gully or Septic — How to Tell

Published October 2026 · By Jack Schroeder, BPC Registered Plumber

The Three Usual Causes

Sewer gas is mostly hydrogen sulphide — the rotten-egg smell — and it only reaches your nose if there's an opening between the sewer and the air. In a healthy system every opening is sealed by water sitting in a trap, or vented above the roofline. So the question is always: where has the seal broken?

1. The overflow relief gully trap has dried out or blocked

That grated drain near the laundry or bathroom wall is your overflow relief gully (ORG), and underneath it is a water trap that keeps sewer gas in the pipe. If nothing drains into it for a while — the laundry trough that used to feed it was removed, or it's just a dry spell — the trap evaporates and the pipe breathes straight out through the grate. Leaves and silt in the gully can do the same by holding the trap open. This is the most common cause of a yard smell that comes and goes, and it costs nothing to test.

2. A cracked, broken or root-damaged sewer pipe

Most homes in Boronia, Bayswater, Ringwood, Croydon and the older parts of the east still run on earthenware sewer pipe laid in short lengths with a joint every metre or so. Tree roots find those joints, open them up, and the pipe starts leaking wastewater into the surrounding soil. You get a smell that's strongest right above the pipe run (usually along the side of the house, between the ORG and the boundary), a patch of grass that's greener or soggier than the rest, and sometimes ground that's slowly sinking. It's worse after rain because saturated soil pushes the gas up, and worse on hot days because warmth drives it off faster. A broken inspection shaft or a cracked ORG riser does the same thing closer to the surface.

3. The septic tank or trenches are full or failing

If you're in Kalorama, Olinda, Monbulk, Emerald or anywhere else in the Ranges or Yarra Valley that isn't yet on the sewer, a sewage smell outside is usually the septic system telling you something: the tank is due for a pump-out, the effluent trenches are waterlogged and surfacing, or the house drain between the house and the tank is blocked and backing up. Smell plus soggy ground over the trenches, slow drains inside, or a gurgling toilet is the classic combination. Our blocked septics page covers which part of a septic problem is a drain plumber's job and which needs a pump-out contractor.

The less common ones

• A broken or disconnected vent pipe. The sewer vent should finish above the roof. If it's cracked low down, or someone removed it during a reno, sewer gas vents at nose height.
• A stormwater pit holding stagnant water. Smells swampy rather than like sewage, and it's usually the pit nearest a downpipe with no outlet. Not dangerous, but the pit needs cleaning and a proper outlet.
• The neighbour's problem. Sewer gas travels. If the smell is strongest at the fence line and your own checks come up clean, say so when you call — it changes where we look.

Narrow It Down Before You Call

Two quick checks worth doing yourself: the bucket test on the ORG (if the smell drops right off within a day, that was it), and a slow walk along the sewer run on a warm afternoon — the smell will peak over the leak. Don't dig, and don't lift inspection-shaft lids if they're below ground level; sewer gas in a confined space is a genuine hazard.

How We Find It Without Digging Up the Yard

1. CCTV camera. A self-levelling colour camera goes through the sewer from the ORG or inspection shaft, with an on-screen distance counter. Cracked joints, root intrusion, displaced pipe and standing water show up on screen, and the distance reading tells us exactly where in the yard the damage sits. This is our CCTV drain inspection — you get the footage and a written report.
2. Smoke test. When the camera shows a sound pipe but the smell persists, we fill the drain with harmless, visible smoke. Wherever it escapes — a hairline crack, a broken vent, a disconnected fitting under the deck — is where the gas has been getting out. It finds the leaks a camera can't see from inside. Details on the drain smoke testing page.
3. Pipe locating. A sonde in the camera head lets us mark the exact position and depth of the fault on the ground, so if a repair is needed it's one small hole in the right place, not a trench along the side of the house.

The fix depends on what we find. Roots in a joint that's otherwise sound: jet the roots out and reline that section so it's sealed for good. A crushed or collapsed length: dig and replace just that piece. A dry trap: restore the water seal and sort out why it dried. A failing septic: pump-out and, if the trenches are done, a council-permitted repair or replacement by a septic installer — we'll tell you straight if that's the case rather than clearing a drain that isn't the problem.

What It Costs

Fixed price before work starts, no call-out fee in business hours. A CCTV inspection of the sewer with footage and a written report is $300–$400; a smoke test is in the same range and is often done on the same visit. If the camera also finds a blockage, a main-line jet and camera together is typically $500–$750. Relining or replacing a damaged section is quoted from the footage before you commit — you'll know the length, the depth and the price. See the cost guide for the full breakdown.

Is It Mine or the Water Authority's?

On a sewered block, the house drain from your fixtures to the connection point near the boundary is the property owner's; the main in the street and the connection beyond that point belong to Yarra Valley Water (most of Knox, Maroondah, the Ranges and the Yarra Valley) or South East Water (Rowville, Wantirna, Knoxfield, Upwey and south). A leak that smells in your yard is almost always on your side, but the camera footage settles it — and if the fault turns out to be past the boundary, that footage is what you hand to the authority. On a septic block the whole system is the owner's, with repairs and replacements needing a permit from the council (Yarra Ranges manages a very large number of on-site systems). Renters: a sewage smell outside that points to a leaking sewer counts as a repair the rental provider has to arrange — the who-is-responsible guide explains the process.

Why It Gets Worse in Warm Weather

Heat drives gas out of wastewater and soil faster, and a dry spell is exactly when gully traps evaporate — so late spring and summer are peak season for "the yard suddenly smells" calls across Melbourne's east. Drier ground also pulls tree roots further toward the one reliable source of moisture on the block, which is a leaking sewer joint. If the smell has shown up for the first time this season, the odds are good it's a trap that's dried out or a root-opened joint that's been quietly weeping for a while and has only now become noticeable. Either way, the sooner the camera goes down the smaller the fix tends to be.

Frequently Asked Questions