Quick Answer
A blocked drain is almost always caused by one of five things: a build-up of hair and soap scum, cooking grease and food scraps, tree root intrusion, foreign objects, or a damaged pipe. Most blockages can be cleared quickly — but knowing what’s causing the problem makes all the difference.
If your sink is draining slowly, your shower is pooling with water, or your toilet is gurgling — you’ve got a blockage somewhere in the system. It’s one of the most common plumbing problems Australian homeowners deal with, and the good news is that it’s almost always fixable.
This guide walks you through the five most common causes of blocked drains, the warning signs to watch for, and exactly when to call in the professionals.
The 5 Most Common Causes of Blocked Drains
1. Hair and Soap Scum (Bathroom Drains)
Hair is the number one culprit in bathroom drains. Every time you shower or wash your hair, loose strands make their way into the drain and start to bind together with soap residue. Over time, this creates a sticky, matted plug that traps everything else that goes down — and it gets worse fast.
You’ll notice it first as slow drainage, then as water pooling around your feet in the shower. Left unchecked, it leads to a complete blockage.
2. Grease and Food Scraps (Kitchen Drains)
In the kitchen, the main offenders are cooking fats, oils, and grease — known in the plumbing trade as “FOG”. When hot grease is poured down the sink it seems harmless, but as it cools inside the pipe it solidifies and sticks to the pipe walls. Food particles, coffee grounds, and soap then cling to it, gradually narrowing the pipe until it blocks completely.
3. Tree Root Intrusion
Tree roots are naturally drawn to the moisture and nutrients inside underground pipes. Over time, fine root fibres find tiny cracks or joints in older pipes and begin growing inside them — eventually causing a partial or complete blockage, or even cracking the pipe itself.
If you have large trees near your home and notice recurring slow drains or gurgling sounds from multiple fixtures at once, tree roots may be the cause. This needs professional assessment with a CCTV drain camera.
4. Foreign Objects
Children’s toys, cotton buds, sanitary products, “flushable” wipes, and dental floss are among the most common items retrieved from blocked drains. Despite what the packaging says, most “flushable” products don’t break down properly in Australian plumbing systems.
5. Damaged or Collapsed Pipes
In older homes, pipes can corrode, crack, shift, or collapse over time — particularly clay or cast iron. If blockages keep coming back despite clearing them, a damaged pipe may be the underlying cause.
Warning Signs You Have a Blocked Drain
- Water draining slowly or not at all
- Gurgling or bubbling sounds from drains, toilets, or pipes
- Unpleasant smells coming from drains
- Water backing up in a sink, shower, or toilet
- Multiple drains slow at the same time — often a main sewer line blockage
- Water pooling around floor drains or in the garden near your pipes
Can You Unblock a Drain Yourself?
For minor blockages — particularly hair in a shower drain or a slow-draining bathroom sink — try these before calling a plumber:
- Remove visible debris: Unscrew the drain cover and pull out any hair or gunk by hand. Always wear gloves.
- Boiling water: For kitchen grease blockages, slowly pour boiling water down the drain in two or three stages.
- Baking soda and vinegar: Pour half a cup of baking soda followed by half a cup of white vinegar. Wait 15 minutes, then flush with hot water.
- A plunger: Works well for toilet and sink blockages. Create a firm seal and pump with steady, even pressure.
When to Call a Licensed Plumber
- The blockage keeps coming back after you've cleared it
- Multiple drains in your home are slow or blocked at the same time
- You can hear gurgling noises from drains or toilets
- There's a persistent foul smell that won't go away
- You suspect tree roots or a damaged pipe
- Water is backing up rather than draining at all
- There's sewage smell coming from your yard
How to Prevent Blocked Drains
- Fit drain guards in shower and bath drains — empty them weekly
- Never pour cooking oil or grease down the sink — dispose of it in the bin
- Run hot water down the kitchen drain after washing up — helps flush residual grease
- Only flush toilet paper — wipes — including "flushable" ones — go in the bin
- Have drains professionally inspected every 1–2 years — especially with large trees near your sewer line
Frequently Asked Questions
Can a blocked drain fix itself?
Minor blockages sometimes clear on their own, but most don't. Address slow drainage early — a full blockage is much harder and more expensive to clear.
How much does it cost to unblock a drain in Australia?
A standard drain unblock typically costs between $150 and $400, depending on severity, location, and equipment needed.
Why does my drain keep blocking in the same spot?
Repeated blockages usually point to an underlying issue — pipe collapse, tree root intrusion, or incorrect pipe grade. A CCTV inspection will find the cause.
Is it safe to use chemical drain cleaners?
Use them sparingly. They're harsh on pipes and bad for the environment. Avoid entirely on older or PVC pipes.
How do I know if it's a main sewer line blockage?
If multiple drains are slow or backing up simultaneously — toilets, sinks, shower — it's likely a main sewer issue. Call a licensed plumber urgently.
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Written and reviewed by the licensed plumbers at Mr. Clog.