Drain Fly Infestations in Sydney: Causes and How to Get Rid of Them

You spot them on the bathroom wall or hovering around the kitchen sink—tiny, fuzzy, moth-like flies that seem to appear from nowhere. Drain flies are a common Sydney nuisance, and while they don't bite or spread disease the way some pests do, their presence is telling you something: there's organic gunk building up in a drain somewhere.

The good news is that drain flies are very beatable, because their weakness is obvious—remove their breeding material and they can't survive.

What Are Drain Flies?

Drain flies—also called moth flies or sink flies—are small (2–5 mm), grey to black, with fuzzy, moth-like wings held roof-like over the body. They're weak flyers that move in short hops and tend to rest on walls near their breeding site.

• Harmless but persistent: They don't bite, but large numbers are unpleasant and signal a hygiene issue.
• Fast breeders: Eggs to adult in about one to three weeks, so numbers build quickly.
• Homebodies: They rarely stray far from where they hatched—useful for finding the source.

Why You Have Them: The Breeding Sites

Drain flies lay eggs in the gelatinous organic film that coats the inside of neglected drains and pipes. Common Sydney sources:

• Bathroom and shower drains—hair, soap scum, and skin cells build a rich film.
• Kitchen sink and disposal—food residue and grease.
• Floor drains in laundries, garages, and bathrooms that rarely get water.
• Rarely used drains—a guest bathroom or a floor waste where the trap has dried out.
• Overflow pipes and slow drains holding standing water.
• Outdoor drains and blocked gutters.

👉 Related: Silverfish in Sydney Bathrooms and Keeping Pests Out of Your Kitchen in Summer.

Find the Source First

Before treating anything, locate the breeding drain. The tape test is simple and reliable:

• Dry the area around each suspect drain.
• Tape clear sticky tape over the drain opening at night, leaving a small gap for air (a full seal can dry the trap).
• Check in the morning—flies stuck to the underside of the tape confirm that drain is breeding them.
• Repeat for each drain to find every source.

How to Get Rid of Drain Flies

Step 1: Clean the Drain Mechanically

This is the step that actually works. Physically scrub the organic film from inside the pipe:

• Use a stiff drain brush or a straightened wire brush to scrub the inside walls of the pipe.
• Remove and clean the drain cover and the P-trap where accessible.
• Pour boiling water down the drain to loosen residue before and after scrubbing.

Step 2: Break Down the Remaining Film

• An enzyme-based drain cleaner (bio/gel type) digests the organic matter drain flies breed in—more effective here than harsh chemical cleaners.
• A baking soda + vinegar flush, followed by hot water, helps loosen buildup.
• Avoid relying on bleach alone—it can clear the surface but often doesn't remove the film clinging to the pipe walls.

Step 3: Deal With the Adults

• Once breeding sites are cleaned, the adults die off within days—they're short-lived.
• Wipe or vacuum resting flies off walls; a simple apple-cider-vinegar trap catches stragglers.
• Don't bother fogging the room—it does nothing about the source.

Keeping Them From Coming Back

• Clean drains regularly, before film has a chance to build.
• Run water through rarely used drains weekly so traps don't dry out.
• Fit drain covers and clear hair and food debris often.
• Fix slow drains and leaks that leave standing water.
• Keep the kitchen sink and disposal free of grease and food residue.
• Clear gutters and outdoor drains.

When to Call a Professional

Most drain fly problems are DIY-solvable. Call a professional when:

• Flies keep returning after you've cleaned every drain you can find—the source may be a hidden or inaccessible pipe, a broken drain, or a sewer issue.
• You manage a rental, café, or food business where hygiene standards apply.
• The infestation is heavy or spread across multiple rooms.
• You suspect a cracked or leaking pipe feeding the problem inside a wall or subfloor.

A professional can identify hidden breeding sources and treat them, and flag any plumbing fault behind a persistent problem. 👉 Read When to Call Professional Pest Control.

Final Thoughts

Drain flies look like a fly problem but they're really a drain problem. Find the breeding drain with the tape test, scrub out the organic film mechanically, and break down what's left with an enzyme cleaner—the adults die off on their own once the source is gone.

Keep drains clean and running and they won't come back. If they keep returning despite a thorough clean, there's a hidden source or a plumbing fault worth getting a professional to find.

👉 Persistent problem? Pest Control Sydney Wide