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What actually causes a blocked drain, and can I clear it myself?

The short answer

Most household blockages are fat, hair or foreign objects near the fixture and can be cleared with a plunger or hand snake. Recurring blockages across multiple fixtures usually mean tree roots in the sewer line, which needs a camera and a professional.

TradieAnswers editorial · written in-house and sourced to the authorities listed below

Checked against 3 published sources, listed in full at the end of this page. How we source this

The single most useful diagnostic is how many fixtures are affected. One slow basin is a local problem. Several fixtures backing up, or a gully outside overflowing, means the blockage is downstream in the main line and nothing you do at the basin will reach it.

Common causes, roughly in order

  • Fat and grease congealing in kitchen waste lines. The most common kitchen blockage, and cumulative rather than sudden.
  • Hair and soap binding together in bathroom wastes.
  • Wet wipes and sanitary products, which do not break down regardless of what the packet claims.
  • Tree roots entering sewer lines through joints, which is the usual cause of a recurring whole-house problem, particularly in older clay pipes.
  • Collapsed or misaligned pipe, most often in older properties or after ground movement.

Worth trying yourself

  1. A plunger, used properly, cover the overflow and get a real seal before pumping.
  2. A hand-operated drain snake for a fixture waste.
  3. Hot water and a mechanical clean for a slow kitchen drain caught early.

When it needs a professional

Repeated blockages, multiple fixtures affected, or sewage surcharging outside all point at the main line. The right response is a camera inspection so the cause is identified rather than guessed at, because root intrusion, a collapse and a simple grease blockage need three different fixes and cost very different amounts.

Who does this work

You may need a licensed plumber

Because clearing a trap or a floor waste yourself is fine, but anything beyond the fixture, including the sewer connection, is licensed plumbing work.

This is licensed work in every Australian state and territory. Check a licence in your state

Where are you?

Who regulates plumbing and gas in your state

Trade regulation in Australia is state law, so the body that answers this depends on where the work is. Pick yours.

New South Wales — NSW Fair Trading
Plumbing, draining and gasfitting are separate licence classes. Work must be notified and is subject to inspection. How to check a licence in NSW
Victoria — Building and Plumbing Commission
Licenses and registers plumbers across separate classes of work. Licensed plumbing work generates a compliance certificate you should be given. How to check a licence in VIC
Queensland — Queensland Building and Construction Commission
Licenses plumbers and drainers. Notifiable work must be submitted to the QBCC after completion. How to check a licence in QLD
Western Australia — Building and Energy
Plumbing contractor and tradesperson licensing, and gasfitting authorisation, sit with the same body. How to check a licence in WA
South Australia — Consumer and Business Services
Plumbing and gas fitting are treated as building work but need specific plumbing, gas fitting and electrical licence classes, known as PGE. How to check a licence in SA
Tasmania — Consumer, Building and Occupational Services
Plumbers, gasfitters and auto gasfitters are licensed occupations, all searchable in one place. How to check a licence in TAS
Australian Capital Territory — Access Canberra
Plumbers, drainers and gasfitters are licensed construction occupations in the ACT. How to check a licence in ACT
Northern Territory — Plumbers and Drainers Licensing Board
Holds the list of currently licensed plumbers and drainers. The board can also confirm a licence by phone if a name is not appearing. How to check a licence in NT

Before you call anyone

  1. Check whether one fixture is affected or several. Several at once usually means the problem is in the main drain rather than under a sink.
  2. Look for an overflow relief gully outside, generally a grate lower than the lowest indoor fixture. If it is surcharging, stop using water inside.
  3. Note whether the blockage returns after being cleared, and how quickly. A blockage that comes back every few months usually means roots or a broken pipe rather than what went down the sink.
  4. Ask for a camera inspection if it is a repeat, and ask to see the footage rather than just be told what it showed.

Sources

Every source below was fetched and its response recorded on the date shown. Where a government site blocks automated clients, that is noted rather than reported as a successful check.

  1. National Construction Code · Australian Building Codes BoardChecked 2026-08-10 · responded 200 OK
  2. Building and Plumbing Commission · Building and Plumbing Commission VictoriaChecked 2026-08-10 · live, but blocks automated clients so the specific page could not be confirmed
  3. NSW Fair Trading · NSW Fair TradingChecked 2026-08-10 · responded 200 OK

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