Blocked Drains & Pipe Relining

Why is my toilet or drain gurgling, and is it a warning sign?

A gurgle is air being forced through water where it should not be, and it is usually the first sign of a drain that is starting to fail.

Written by Mathew Veselcic, licensed plumber and owner of Voyager PlumbingPublished 19 October 2026 · 9 min read

The short answer

A gurgling toilet or drain usually means air is being forced through the water in a trap, because a pipe is partly blocked or short of air from a blocked vent. One gurgling fixture is often a local blockage. Several gurgling at once, or water in the outside gully, points to the main line and needs a plumber soon.

Key takeaways

• Sydney Water lists a gurgling toilet or floor drain as a warning sign of a wastewater blockage.
• One gurgling fixture usually means a local blockage; several at once points to the main line.
• Water rising in the outside gully, plus gurgling, means stop using water and call a plumber.
• Tree roots are the most common cause of blockages, and North Shore streets are full of the species Sydney Water warns about.
• If neighbours have the same problem, report it to Sydney Water on 13 20 90.

Jump to a section8

• What a gurgling toilet or drain is telling you
• The four common causes of a gurgling toilet or drain
• Gurgling toilet or drain: what each pattern usually means
• Why gurgling drains are common in older North Shore homes
• What to check yourself before calling a plumber
• How a plumber diagnoses and fixes a gurgling drain
• Who pays to fix a gurgling drain in Sydney
• Frequently asked questions

What a gurgling toilet or drain is telling you

A gurgle is the sound of air being pulled or pushed through water where it should not be. Drain pipes are meant to carry water downhill with air moving freely alongside it. When something slows the water down or starves the pipe of air, the air finds another way through, and the easiest way is often back up through the water sitting in your toilet bowl or the trap under a floor drain.

Sydney Water treats it as a warning sign. Its page on wastewater blockages lists a toilet or floor drain that makes gurgling noises alongside slow sinks, a toilet that will not flush properly and wastewater overflowing from the gully near the outside tap, under the heading “Don’t ignore these signs”.

A gurgle on its own is not an emergency. It is an early signal, and it usually arrives before the blockage gets bad enough to back up. Acting at the gurgle stage is generally a much smaller job than acting once a toilet has overflowed.

The four common causes of a gurgling toilet or drain

Almost every gurgle comes back to one of four things. Which one it is decides whether it is a five minute fix or a job for a drainer.

1. A partial blockage in the drain

Wipes, fat, hair or roots narrow the pipe so water cannot get past freely. As water squeezes through the gap, it drags air with it, and that air bubbles back up through the nearest trap. This is the most common cause, and it gets worse over time rather than better.

2. A blocked or missing vent

Every drainage system needs air. Vent pipes, usually the open pipes that run up past the roofline, let air into the system so draining water does not suck the water out of your traps. If a vent is blocked by leaves, a bird’s nest or debris, the pipe pulls air from the next easiest place: your toilet bowl or a floor waste.

3. A blockage in the main line

If the blockage is further down, in the line that carries everything from the house to the sewer, every fixture is affected. You will hear gurgling in one fixture when another drains, and the lowest fixture in the house usually suffers first.

4. Water backing up from the sewer in heavy rain

Sydney Water says heavy rain is a common reason for wastewater overflows, and lists stormwater pipes connected directly to the wastewater system as one of the causes. When the sewer is running full, your drains have less room to empty, and gurgling during or straight after a storm is often the first sign.

Gurgling toilet or drain: what each pattern usually means

Where and when you hear the gurgle narrows the cause down quickly. Use this as a first read, not a diagnosis.

What you notice	Most likely cause	How urgent
One sink or shower gurgles, everything else is fine	A partial blockage in that fixture’s own pipe or trap	Low. Clean the trap and watch it
The toilet gurgles when the bath, shower or washing machine empties	A shared pipe that is partly blocked, or a vent problem	Medium. Book it in before it gets worse
Several fixtures gurgle and drain slowly at once	A blockage in the main line out of the house	High. This is how backups start
Gurgling plus water rising in the outside gully	A main line or boundary trap blockage	High. Stop using water and call a plumber
Gurgling only during or after heavy rain	A full sewer, or stormwater getting into the wastewater pipes	Medium. Worth a camera inspection
A floor drain that is rarely used smells of sewage	The water seal in the trap has dried out	Low. Pour water into it

That last row is easy to fix. A floor waste that never gets water can lose the seal that keeps sewer gas out. NSW plumbing rules recognise this: the Building Commission’s advisory note on tundish installation quotes the requirement that floor waste gullies or similar traps subjected to infrequent use be provided with an approved means of maintaining their water seals. In an older house, a bucket of water every few weeks does the same job.

Why gurgling drains are common in older North Shore homes

Three things about the North Shore make gurgling more likely than in a newer suburb.

Big trees over old pipes

Ku-ring-gai, Willoughby and the rest of the North Shore are known for their established tree canopy, and much of it grows above older sewer lines. Sydney Water says tree roots that find their way into the pipes are the most common cause of blockages. Its list of trees to avoid planting near wastewater pipes includes figs, jacarandas, gums, liquidambars, brush box and lilly pillies, which reads like a walk down most North Shore streets. It recommends keeping new trees at least 6 metres from wastewater pipes.

Roots rarely block a pipe overnight. They get in at a joint, thicken, and catch paper and wipes, which is exactly the slow narrowing that causes a gurgle long before a full blockage.

Boundary traps and their vents

Many older homes have a boundary trap, a water seal near the front boundary between the house drain and the sewer. NSW Fair Trading’s technical note on air admittance valves says drains that incorporate a boundary trap must have an open vent both upstream and downstream of it. If either vent is damaged or blocked, the system loses air and fixtures start to gurgle. Our guide explains what a boundary trap does and how to find yours.

Renovations that changed the venting

A new bathroom or laundry often means new pipe runs tied into an old system. Some vents can be replaced by an air admittance valve, a one way valve that lets air in without a pipe through the roof. The same NSW technical note says these must be correctly sized, accessible, and installed upright, and that most non-compliant installations it identified were about location, termination or sizing. A poorly placed valve can leave a fixture short of air.

What to check yourself before calling a plumber

A few checks take ten minutes and tell you a lot. None of them involve chemicals or opening up the drain.

1. Find out how many fixtures gurgle. Run the basin, then the shower, then flush the toilet, and listen at the others. One fixture points to its own pipe. Several point to the main line.
2. Look at the outside gully. This is the grated drain near an outside tap, called an overflow relief gully. Sydney Water describes overflow from the gully near your outside tap as a sign of a blockage. If it is full or spilling, stop using water. Our guide to the outside gully shows what it should look like.
3. Check whether neighbours have the same problem. Sydney Water says that if neighbours are also affected, or there is a continuous overflow when no water is being used, the issue is probably in its wastewater system. Report that on its faults line, 13 20 90, which runs 24/7.
4. Clean the trap on a single slow fixture. Hair and soap in a basin or shower trap are a common, simple cause. For a toilet, our step by step guide on how to unblock a toilet covers the safe options.
5. Top up any dry floor wastes with a bucket of water if the problem is a smell rather than a sound.

What not to do

• Do not pour caustic drain cleaner into a toilet or a drain that is backing up. If it does not clear, the next person to open the pipe is working with it.
• Do not keep flushing a toilet that gurgles and rises. Each flush adds water to a pipe that cannot take it.
• Do not ignore it because it went quiet. A partial blockage can stop gurgling for a while as it shifts, then come back worse.

If the toilet itself is the problem, such as a cracked pan, a loose seal at the floor or a cistern that keeps running, that is a fixture job rather than a drain job. We handle toilet repairs on the North Shore as well.

How a plumber diagnoses and fixes a gurgling drain

A good diagnosis finds where the air or water is getting stuck, not just clears the symptom. The usual order:

1. Locate the blockage by checking which fixtures are affected and opening the nearest access point, often the boundary trap riser or an inspection opening.
2. Clear it with an electric eel, a high pressure jetter, or a root cutter if roots are the cause.
3. Run a camera through the line to see whether it blocked because of something flushed, or because roots or a broken joint will keep catching debris.
4. Check the vents if the drains are clear but the gurgle remains, including the roof vent and any air admittance valves.

Indicative costs to judge a quote against

Job	Indicative range across Sydney
Simple drain clear	$150 to $400
Blocked main sewer line	$300 to $600
High pressure jetting	$250 to $600
CCTV drain inspection	$250 to $550
Root cutting	$400 to $1,200

These are indicative ranges to judge a quote against, not a quote. Access, depth and what the camera finds all move the price. Vent repairs are priced on what is found, because a cleared roof vent and a re-plumbed bathroom are very different jobs.

When the fix means digging

If the camera shows a broken pipe under a large tree, the repair can involve excavation. In Ku-ring-gai, the council’s tree and vegetation preservation controls count changing ground levels in a tree’s root zone by excavation or trenching, and severing roots of 50mm or more, as injuring a tree. Injuring a protected tree is prohibited without council’s written consent unless an exemption applies, so check before anyone digs near a big tree.

Who pays to fix a gurgling drain in Sydney

On most houses, a gurgling drain is a private plumbing problem. Sydney Water’s guidance on maintaining your service says property owners are responsible for their wastewater pipes up to where they connect to its system, including clearing tree roots that enter the junction at the point of connection.

Where the problem is	Who is responsible
Fixtures, traps and pipes inside the house	Property owner
Vents and air admittance valves	Property owner
Private line and boundary trap	Property owner
Roots entering the junction at the point of connection	Property owner
Sydney Water’s wastewater network	Sydney Water

If a licensed plumber finds the blockage is in Sydney Water’s pipes and reports it, Sydney Water’s blockages page says you can claim reasonable out-of-pocket expenses, with the job number the plumber gets from 13 20 90 included in the claim.

Preventing the next gurgle

• Only flush the 3 Ps: pee, poo and toilet paper. Sydney Water says non-flushable wet wipes contribute to 75% of wastewater blockages.
• Keep fat and oil out of the sink. Wipe pans and bin it.
• Keep the outside gully and roof vents clear of garden beds, leaves and debris.
• Know where your pipes run before planting a tree, and keep it well clear.

If the gurgling has started and you want it found rather than guessed at, our drain unblocking service for North Shore homes will clear the line and show you the camera footage. For anything else in the house, Voyager’s trusted plumbers across the North Shore are a phone call away.

Frequently asked questions

In short

A gurgling toilet or drain is air being forced through water, usually because a pipe is partly blocked or a vent is not letting air in. One fixture is a local problem; several at once, or water in the outside gully, points to the main line. On older North Shore blocks, tree roots in earthenware pipes are the usual culprit, so get the line cleared and inspected before the gurgle becomes a backup.

Sources

• Sydney Water, Wastewater blockages
• Sydney Water, Avoid wastewater overflows
• Sydney Water, Maintaining your service
• NSW Fair Trading, Technical Note 4/2013, Installation requirements of Air Admittance Valves (PDF, May 2021)
• Building Commission NSW, Tundish installation advisory note
• Ku-ring-gai Development Control Plan, Part 13 Tree and Vegetation Preservation (PDF)