If you have noticed that your drains develop a stronger or more unpleasant odour after heavy rain, you are not imagining it. This is one of the most telling symptoms of a sewer line problem in Sydney homes, and it is worth taking seriously. Understanding why rain makes drain smells worse will help you explain the issue clearly when you call a plumber and understand what needs to be done to fix it.
Why Rain Makes Drain Smells Worse
There are several mechanisms by which rainfall intensifies drain odours. Most of them point to a problem in the sewer line or stormwater drainage system rather than a surface-level drain issue.
Rising Groundwater and Sewer Line Pressure
Heavy rainfall raises the water table around your property. When this happens, the soil surrounding your sewer pipes becomes saturated with water, which increases the pressure on any cracked, deteriorating, or partially collapsed sections of pipe. This elevated pressure can force sewer gases through existing cracks and out of the pipe at a faster rate than in dry conditions, which is why the odour inside the property intensifies during and after rain. A licensed blocked drain plumber in Sydney can identify pipe cracks and deterioration using a CCTV camera inspection.
Increased Flow Through Partially Blocked Sewer Lines
When it rains heavily, groundwater and stormwater can infiltrate sewer pipes through cracks and poorly sealed joints — a problem known as inflow and infiltration. This additional water increases the volume flowing through the sewer system. In a sewer line that is partially blocked by root intrusion, accumulated debris, or scale buildup, the increased flow disturbs the stagnant material sitting in the blockage and releases gases that had been trapped. This is why a sewer that is manageable in dry conditions can produce a noticeably stronger smell during rain.
Tree Root Intrusion Becomes More Active
Tree roots inside a sewer pipe grow actively after rain because moisture levels increase around the root system. As roots expand and move inside the pipe after a rain event, they disturb accumulated organic matter, releasing decomposition gases in greater quantities than during dry periods. If the sewer smell in your property is consistently worse after rain and your property has large or mature trees, root intrusion is a strong candidate for the cause. Related: Why Does My Shower Drain Smell?
Stormwater Drains Connected to the Sewer System
In some older Sydney properties, stormwater pipes were incorrectly connected to the sewer system at some point in the property’s history. During heavy rain, large volumes of stormwater flood into the sewer system through these illegal connections. This overwhelms the sewer system’s capacity and can force sewer gas back through drain openings inside the property. If your drain smells are specifically triggered by heavy rain and you are in an older Sydney suburb, this is a possibility worth investigating.
Blocked Stormwater Drains Backing Up
If outdoor stormwater drains on your property are blocked, rainwater has nowhere to go and backs up across the yard and around the building’s foundations. When this water contacts deteriorating sewer pipes below the surface, it can force gas up through any available opening. Clearing blocked stormwater drains and ensuring they are properly separated from the sewer system resolves this pathway. A licensed commercial plumber in Sydney can assess whether the stormwater and sewer systems on your property are correctly separated.
Is It Just the Outside Drains or Inside Too?
Pay attention to where the rain-related smell is strongest. If the odour is primarily from outdoor drains and the yard area, blocked stormwater drains or surface drainage problems are likely involved. If the smell is coming from inside the property — through bathroom floor drains, the shower, the toilet, or the kitchen sink — the problem is in the sewer line itself and requires a professional inspection.
A smell that is simultaneously present from both indoor and outdoor drains after rain typically indicates a more significant sewer line issue, such as a pipe collapse or a severe root intrusion that is backing up the entire system. See also: 7 Plumbing Emergencies That Cannot Wait.
What to Do When Your Drains Smell Worse After Rain
Because rain-related drain smells almost always indicate a problem in the sewer or stormwater system rather than a surface drain issue, the appropriate response is to call a licensed plumber rather than attempt a DIY fix.
CCTV Drain Inspection
A CCTV camera inspection of the sewer line is the most reliable way to identify the exact cause of rain-related drain odours. The camera travels through the pipe and records the internal condition, revealing root intrusion, cracks, collapsed sections, offset joints, and areas of significant scale buildup that are generating odour. Without this inspection, any treatment applied is essentially guesswork. A licensed Sydney plumber can carry out a CCTV inspection and provide a report with findings and recommended treatment.
Root Cutting and Hydro Jetting
If root intrusion is identified, the treatment involves cutting the roots out of the pipe using a root cutting attachment on an electric eel, followed by high-pressure hydro jetting to flush all root material and accumulated debris through the system. Depending on the severity of the root intrusion and the condition of the pipe, pipe relining may also be recommended to seal the cracks through which roots entered and prevent regrowth.
Pipe Relining or Repair
If the inspection reveals cracked, deteriorating, or partially collapsed pipe sections, pipe relining or spot repair is the appropriate treatment. Pipe relining creates a new pipe inside the existing one without excavation, sealing cracks and restoring structural integrity to prevent further inflow, infiltration, and gas escape.
If Your Drains Smell Worse After Rain in Sydney, Call 1300 301 354
All Day Plumbing carries out CCTV sewer inspections and full sewer line remediation across Sydney. If your drains are consistently smelling worse after rain, our licensed team can identify the cause and fix it at the source. Call us on 1300 301 354. We are available 24 hours a day, 7 days a week, and offer same-day service across all Sydney suburbs. Do not let a post-rain sewer smell go uninvestigated — it is a clear sign your sewer line needs professional attention.