A bloke in Cabramatta rang me last summer because his ensuite smelled like a public toilet every morning. Nothing was blocked, and he had already tried bleach and drain cleaner. It took me four minutes to find the cause. The shower in that room had not been used since his son moved out, the floor waste had dried out, and sewer gas was drifting up through an empty trap every night. A bucket of water fixed it.
Smelly drains are one of the most common calls I get across South West Sydney, and most of the causes are cheap to fix. Here they are in the order I see them, and where the line sits between a jug of water and a plumber. If you would rather hand it to a licensed blocked drains plumber, no call-out fee, ring 0455 222 292 and Paul answers.
A sewer smell in the house is almost always a trap that has lost its water seal: it has dried out in an unused shower or floor waste, or a partial blockage downstream is sucking it dry. Refill every drain in the room and see if the smell stays away. If it comes back, or any drain gurgles, the main line needs clearing and a camera inspection.
Smelly drains: where the sewer smell in the house is coming from
Every drain in your house has a trap: the U-bend under a sink, the S or P bend behind a toilet, the gully under a floor waste. The water sitting in that bend is the only thing between your bathroom and the sewer. If that water is gone, or has been sucked out by a blockage further down the line, you smell sewer gas. If the smell is more rotten food than sewage, it is usually gunk in the trap itself.
Dry trap smell: the number one cause of a bathroom drain smell
Water in a trap evaporates. In a Sydney summer an unused floor waste can dry out in two or three weeks. Guest bathrooms, laundries, the downstairs shower in a two-storey place in Prestons where everyone uses the upstairs one: these are the usual culprits. The smell is worst first thing in the morning, because the gas builds up overnight.
The fix is a jug of water down every drain in the room, including the floor waste, once a fortnight. If a room never gets used, add a tablespoon of cooking oil after the water; it floats on top and slows the evaporation. That is the whole repair. If the smell is back within a day or two, the trap is not your problem, and you move to the next suspect.
Drain smells from a dirty trap
Kitchen sink traps collect grease and food scraps. Shower and basin traps collect hair, soap scum and toothpaste, which turns into a black slime that smells like a swamp. This is not sewer gas. It is bacteria eating the build-up, and it gets worse in warm weather.
Half a cup of bicarb soda, a cup of white vinegar, ten minutes, then a minute of hot tap water (not a boiling kettle, which can damage plastic pipe) clears a mild case. For a bad one, unscrew the bottle trap under the sink with a bucket underneath and clean it out, or lift the shower grate and pull out the hair. Skip the caustic drain cleaners: they do not remove the slime, they eat rubber seals, and they make the line dangerous to open afterwards. My post on blocked shower drains has more.
Gurgling drain plus smell: the seals keep disappearing
A gurgling drain is the sound of air moving where water should be. When you flush the toilet and the shower gurgles, or the basin burps while the washing machine drains, something downstream is partly blocked. Water rushing past the blockage creates suction that pulls the water out of nearby traps, so the smell keeps coming back however often you fill them.
This is the combination I take seriously, because it usually means the main line is on its way to blocking. Tree roots are the leading cause around Liverpool and Casula, where the old earthenware pipes are cracking at the joints. The gurgling gets worse over a few weeks, and eventually the toilet stops draining, usually on a Sunday. Caught early, it is a business-hours clear and camera inspection at normal rates. There is more in my posts on blocked drains and when to call and tree roots in sewer pipes.
Sewer smell outside: the overflow relief gully and the boundary trap
The overflow relief gully
The overflow relief gully (ORG) is the grated drain in the ground near the laundry or kitchen wall, with a loose grate on top. It is set lower than every fixture inside on purpose: if the sewer blocks, the sewage lifts the grate and comes up in the garden instead of through your shower drain. The NSW installation rules put the top of the gully at least 150 mm below the lowest fixture inside and 75 mm above the surrounding ground. It has its own trap, normally kept full by the laundry tub draining through it.
Three things go wrong with them. The trap dries out because the laundry no longer drains through it, and you get a whiff near the back door. Someone has paved or built a garden bed over it, so when the sewer blocks the sewage comes inside instead. Or it is sitting full of grey water that will not drain, which means the main line is blocked right now. Pull the grate off and look. Clear water at the bottom is normal. No water means tip a bucket in. Water up near grate level is a same-day call.
The boundary trap
Plenty of older homes around Liverpool, Fairfield and Cabramatta have a boundary trap; the new estates mostly do not. It is a large water-sealed trap buried near the front boundary where your drain meets Sydney Water's main, and its job is to stop gas from the street sewer entering your pipes. When it cracks, loses its cap, or fills with roots, you get a sewer smell on the front lawn that no cleaning inside will shift.
The trap sits on the private side of the connection point, so on most properties it is the owner's to fix. Sydney Water looks after the main and the connection point itself (their fault line is 13 20 90). I confirm which side the fault is on with the camera before anyone spends money. That end of the system is covered on my sewer and stormwater page.
Smells that are not the drain at all
A sewer smell at the base of the toilet that gets worse when you flush is usually the rubber seal between the pan and the drain. Every flush pushes a puff of gas past the perished seal. The pan has to come up to fix it, so it is a toilet repair rather than a DIY job. A rotten egg smell only from the hot taps is the hot water system, not the drains.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Smell in a little-used bathroom, worst in the morning | Dry trap | Jug of water in every drain, repeat fortnightly |
| Rotten food smell from the kitchen sink | Grease and slime in the trap | Bicarb and vinegar, then clean the trap by hand |
| Gurgling when other fixtures drain, smell returns after refilling | Partial blockage in the main line | Book a clear and CCTV inspection this week |
| Smell at the base of the toilet after flushing | Failed pan seal | Plumber to lift and reseal the pan |
| Sewer smell in the yard, gully full of water | Main line blocked | Stop using water, call today |
| Sewer smell at the front boundary | Cracked boundary trap or broken pipe | Camera inspection to confirm whose side it is on |
What it costs to fix
Dry traps and dirty traps cost you a jug of water and ten minutes. For the rest, the Sydney 2026 price guides put a blocked drain clear at $180 to $450 and a call-out fee at $69 to $170. A CCTV inspection is quoted on inspection; I usually bundle it with the clear so we know whether the blockage will come back. Pan reseals, boundary traps and broken lines are quoted on inspection too; what moves the price is access, parts and after-hours. A smell without a blockage is never an after-hours job. Fill the traps, open a window, and ring me in the morning.
When to call a plumber
Call if the smell comes back within days of refilling the traps, if any drain is gurgling, if the overflow relief gully has water sitting in it, if the smell is at the toilet base, or if it is coming from the front boundary. Those all point to a fault in the pipework. If the gully is overflowing, stop using water in the house until it is cleared.
I bring the CCTV camera and the jet blaster on the first visit, so the smell gets found and fixed in one trip. Done right. Done once. Monarch covers South West Sydney and the Macarthur region, from Liverpool and Fairfield across to Camden. Have a look at the blocked drains service page or find your local plumber in Cabramatta, and ring 0455 222 292 when you are ready.
Related guides: blocked shower drain and blocked drain causes and fixes.
Need it sorted? Ring 0455 222 292 any hour or book online. Monarch Civil & Plumbing is a blocked drains plumber, no call-out fee, across Liverpool, Fairfield, Campbelltown and Camden, and the rate is the same after hours.
Blocked drain in Liverpool, Fairfield or Campbelltown?
The local pages say what is usually wrong in your suburb and what Paul does about it: blocked drains Liverpool, blocked drains Fairfield and blocked drains Campbelltown. Everywhere else within about 20 km of Austral, the blocked drains page covers how a job runs and service areas lists every suburb. No call-out fee anywhere, and the camera goes down before anything is quoted.
Frequently asked
Why does my bathroom drain smell like sewage in the morning?
Because the water seal in a trap has evaporated overnight, usually in a floor waste or a shower that does not get used. Sewer gas builds up while the house is closed and hits you when you open the door. Pour a jug of water down every drain in the room, including the floor waste, and repeat every couple of weeks. If the smell returns within a day or two, something downstream is pulling the seal out and you need the line checked.
Is sewer gas in the house dangerous?
At the levels you get from a dry trap it is unpleasant rather than dangerous, though headaches, nausea and irritated eyes are common if you live with it. A strong, constant smell is different: it usually means a broken pipe or a blockage, and methane from a sewer is flammable. Open the windows, keep flames away from open drains, and treat a strong smell as a same day call rather than a weekend project.
What does an overflow relief gully do and why is mine smelly?
The overflow relief gully is the grated drain outside near the laundry, set lower than every fixture inside. If the sewer blocks, the grate lifts and the sewage comes up in the garden instead of your shower. It smells for one of two reasons: its trap has dried out because the laundry no longer drains through it, or the main line is blocked and sewage is sitting in it. Lift the grate and look. Dry means a bucket of water; full means a plumber today.
Who is responsible for a boundary trap in Sydney?
In most cases the property owner. The boundary trap sits on the private side of the connection point to the Sydney Water main, and Sydney Water is only responsible for the main and the connection itself. If a smell at the front boundary turns out to be a cracked trap, a missing riser cap or roots in the shaft, the repair is yours. A camera inspection confirms which side the fault is on before anyone spends money.
Will bleach or drain cleaner get rid of a sewer smell?
No. Bleach and caustic cleaners do not put water back in a dry trap and they do not clear a partial blockage in the main line, which are the two real causes. They will mask the smell for a day, eat the rubber seals in your traps, and make the pipe dangerous to open later. Use water for a dry trap, bicarb and vinegar for a slimy trap, and a plumber with a jet blaster for anything that gurgles.
Why does my drain gurgle when the toilet flushes?
Gurgling is air being pulled through a trap because water cannot flow freely downstream. A partial blockage, usually tree roots or a build up of grease and wipes, forces the flush to draw air back through the nearest drain. It is an early warning: the blockage will keep growing until the toilet stops draining altogether. Booking a clear while it is only gurgling keeps it a business hours job at normal rates.
Licensed plumber and drainer based in Austral, 13+ years in the trade, working across Liverpool, Fairfield and the Macarthur region. About Paul