Treat this smell seriously until you know which one it is. One of the two causes is a fire and health hazard that needs immediate action. The other is unpleasant but rarely dangerous. The good news is that telling them apart is usually quick.
Gas leak or sewer gas: 2-minute triage
| Clue | Points to gas leak | Points to sewer gas |
|---|---|---|
| Where is it strongest? | Near the cooktop, heater, meter, bottles or gas hot water unit | Near a floor waste, bathroom, laundry or a rarely used drain |
| When do you notice it? | Constant, or when an appliance runs | Worse after rain, in the morning, or in unused rooms |
| Any hissing sound? | Possible near a fitting or the meter | No |
| Does running water help? | No change | Often disappears after pouring water into the floor waste |
| Gas appliances in the house? | Yes, or LPG bottles outside | May have no gas at all |
| Your next move | Ventilate, no flames or switches, turn gas off at meter or bottle, leave, call from outside | Pour a bucket of water into floor wastes, monitor |
How do I tell if the rotten egg smell is a gas leak?
Natural gas and LPG are odourless, so an odorant called mercaptan is added specifically to smell like rotten eggs and warn you. If your home has gas appliances or LPG bottles and the smell is strongest near any of them, treat it as a leak. Do not flick light switches, do not light anything, and do not hunt for the leak with your phone torch pressed against fittings. Open doors and windows, turn the gas off at the meter (quarter-turn valve, handle across the pipe means off) or at the bottles, get everyone outside, and call from there. LPG is heavier than air and pools low, which matters in garages and under floors on sloping Gold Coast blocks. Leak detection and repair on gas lines is licensed gasfitting work, quoted on inspection, and it is one of the jobs we drop other work for. If the smell is strong and you cannot isolate the supply, call the gas emergency line for your distributor first, then us.
What is sewer gas and why does it smell like rotten eggs?
Sewer gas is a mix that includes hydrogen sulphide, the same rotten egg compound. Every drain in your home has a water trap whose whole job is to hold a plug of water between your rooms and the sewer. When a trap dries out, sewer gas walks straight in. The classic offender is the floor waste in a spare bathroom, ensuite or laundry that has not seen water for weeks: the trap evaporates dry, and the smell arrives, often noticed first thing in the morning or after a hot week. The two minute fix is a bucket of water down every floor waste and a run of every rarely used tap and shower. If the smell fades within a few hours, that was it, and a dash of vegetable oil in the floor waste slows future evaporation. Recurring smells despite wet traps point to something structural: a blocked or unvented stack, a cracked pipe under the house, or a failed seal at a toilet base, and that needs a licensed drainer with a camera. A smell that gets worse after heavy rain often implicates the overflow relief gully or a surcharging line.
When is a sewer smell a blocked drain warning?
When it comes with company. Sewer smell plus gurgling toilets, slow drains or wet patches in the yard is the early signature of a partial sewer blockage, and it is much cheaper to deal with at this stage than after a backup. A machine clear runs $250 to $400, a CCTV inspection to find the source runs $200 to $350, and after-hours attendance, if it becomes a backup at night, runs $360 to $560 for the first hour plus the work. The pattern we see is that smell precedes overflow by days to weeks, so a smell that persists after you have wet every trap has earned an inspection.
What should I do right now?
Gas appliances or bottles in the house and the smell is near them: isolate, ventilate, leave, call from outside. No gas in the house or smell is near drains: bucket of water into every floor waste, then reassess in a few hours.
Sources: Petroleum and Gas (Production and Safety) Act 2004 (Qld); Queensland gas distributor emergency guidance; Plumbing and Drainage Act 2018 (Qld) on sanitary drainage and venting.
Rotten egg smell you cannot explain? Call Hills Plumbing & Gas on 0472 657 042 now, day or night, or book through our Emergency Plumbing service page. Licensed for both the gas side and the drain side, Gold Coast wide, QBCC licence 15243081.