Overview of the Problem – Gurgling Drains and Bad Sewer Smell
At a property on Willow Avenue in Cheltenham VIC 3192, our team at Your Choice Plumbers was called out after the owner reported gurgling noises coming from the bathroom drains, even though the fixtures were still draining.
This is exactly the type of issue where homeowners start asking, “why are my drains gurgling but still draining?” or “why does my bathroom smell like sewer even though nothing is blocked yet?” As an experienced plumber Cheltenham, we often find that gurgling drains and bad sewer smells are some of the earliest warning signs that the main sewer drain is starting to block up.
In this case, the client acted early, which was a very good decision. They recognised the warning signs before the blockage became severe enough to stop the plumbing fixtures from working altogether.
Initial Testing and Early Signs of a Sewer Drain Problem
We began by testing the bathroom fixtures and flushing the toilet several times to observe how the drainage system responded under use.
At first:
- the toilet was still flushing
- water was not yet rising in the pan
- the bathroom fixtures were still discharging
However, while the toilet was being flushed, the bathroom drains started gurgling, and a noticeable sewer smell was present.
From our blocked sewer drain experience, this immediately pointed to the start of a main drain blockage rather than an isolated fixture problem.
The reason is simple: when a sewer line starts to partially block, it affects air movement through the drain before it completely stops wastewater flow.
Why Boundary Trap Systems Cause Gurgling Drains When Blocking Up
This property had a boundary trap connection sewer system, which is important to understand in this type of diagnosis.
A boundary trap system normally has two main venting points:
- a downstream vent, typically the top of the boundary trap shaft fitted with a multi-vent cap
- an upstream vent, usually connected to the last fixture on the drain line, often a minimum of 50mm depending on the system size
The boundary trap shaft itself acts as:
- a breather point
- an overflow point
- a relief point in the event of a major blockage
These vent points allow airflow through the drainage system so wastewater can flow freely.
When one side of that airflow path becomes partially or fully restricted, the internal drains begin to gurgle because the system is effectively gasping for air. In some cases, the pressure change can even start siphoning water from fixture traps, which is what allows bad sewer smell to enter the home.
Confirming the Boundary Trap Blockage
After seeing the internal signs, we moved outside and inspected the boundary trap shaft directly.
We removed the UPVC multi-vent cap and immediately confirmed the problem:
- the sewer level inside the shaft was above its normal level
- the water level rose further with each new flush
That told us the blockage was affecting the boundary trap connection itself and restricting airflow through the system.
This explained both key symptoms:
- the gurgling drains
- the bad sewer smell in the bathroom
Clearing the Sewer Blockage
Once the blockage location had been confirmed, we used our high-pressure jet drain cleaning machine to clear the obstruction through the boundary trap connection.
After jetting the blockage, we:
- flushed the toilets repeatedly
- ran the bathroom fixtures
- continued testing the system under flow
This was important because after a sewer blockage starts to build up, waste can remain along the internal walls of the pipe even after the main choke point is broken through. Running the fixtures and flushing thoroughly helps push the remaining waste through the system and confirm that the line is fully clear.
Once testing was complete:
- the drain was flowing freely again
- the gurgling noise stopped
- the sewer smell disappeared
Why the Owners Did the Right Thing by Acting Early
This was a very good example of why homeowners should never ignore gurgling drains or bad sewer smells, even if the fixtures are technically still draining.
In many cases, those symptoms mean the drain is in the early stages of blocking up. If left untreated, the system can progress from:
- gurgling drains
- sewer smells
- slow pressure changes
to:
- full sewer blockage
- overflowing boundary trap
- inability to use toilets, showers, basins, and other fixtures
Because the owner acted early, the problem was cleared before it reached that stage.
Outcome and Client Result
- early-stage sewer blockage identified
- boundary trap shaft checked and confirmed backing up
- high-pressure jetting used to clear the blockage
- internal fixtures re-tested under flow
- gurgling drains eliminated
- bad sewer smell resolved
The owner was pleased to understand what the warning signs meant and relieved they had acted before the issue became a full drainage emergency.
Compliance & Prevention Insight
This case highlights why gurgling drains and bad sewer smell should be treated as early warning signs of a blocked sewer drain rather than as minor annoyances.
When airflow through the sewer line becomes restricted, the drainage system starts showing pressure-related symptoms before wastewater stops moving completely. That is why strange drain noises and smells often appear before a full blockage.
Recognising those signs early and calling a plumber promptly can prevent much bigger problems, including sewer overflows and loss of use of household fixtures.
Compliance with Australian Standards
All work carried out by Your Choice Plumbers is completed in accordance with the relevant requirements of AS/NZS 3500.2:2021, helping ensure drainage systems are diagnosed and cleared correctly.
In simple terms, this means your sewer drain is assessed properly so the system can vent, discharge, and protect the home the way it was designed to.
Our Home Plumbing Experts Approach
As part of our Home Plumbing Experts approach, we focus on both repair and prevention.
We offer a Complimentary Property Protection Audit as part of our service, assessing key areas like water pressure, hot water temperature, and flexible hoses — helping homeowners stay ahead of potential plumbing failures and avoid unexpected water damage.
Find out more about our blocked drains Melbourne services.