Cleaning · 9 December 2025 · Cameron Veerstagh
Mould and Algae Removal: Solving Darwin's Humidity Problem
Darwin's wet season humidity turns walls, eaves, and concrete green in weeks — learn how to remove mould and algae properly and stop them coming back.
If you've just come back from a wet season trip away and found your house wearing a green coat, you're in good company. Darwin's humidity — sitting at 85–95% from November through to April — creates ideal conditions for mould and algae to colonise virtually any surface that isn't completely sealed or in direct sun. Walls, eaves, fences, concrete paths, even the underside of roof overhangs: all fair game. Knowing how to treat it properly, not just blast it with a hose and watch it come back in three weeks, makes all the difference.
Understanding the Difference: Mould vs Algae
These words get used interchangeably but they're different organisms requiring slightly different approaches. Algae is what gives surfaces that green, slimy coat — it's a plant-like organism that needs light and moisture to grow, so you'll find it on north-facing walls and any surface that gets regular wet-dry cycles. It's slippery and unsightly but not structurally damaging on its own.
Mould is a fungus, and it's the one you need to take more seriously. It appears as black, grey, or greenish-black patches, often with a fuzzy texture, and it tends to spread more aggressively in sheltered spots with less UV exposure — under eaves, in corners, on south-facing walls, inside laundries and bathrooms. On porous surfaces like render or timber, mould can penetrate below the surface layer, meaning a surface clean only buys you time before it re-emerges.
On external surfaces, both algae and mould are primarily a cosmetic and maintenance issue. Indoors, mould becomes a health concern — especially for people with asthma or respiratory sensitivities, which is relevant in a place like Darwin where most households are sealed indoors with air conditioning running for months at a time during the wet.
External Surfaces: The Right Treatment Process
A lot of people grab a pressure washer and go. That removes visible growth but leaves behind spores and roots embedded in porous surfaces — you're back to square one in four to six weeks. The correct process is: kill first, then clean.
Step 1: Apply a biocide or bleach solution
For exterior concrete, render, and masonry, a diluted sodium hypochlorite (bleach) solution at 1:5 to 1:10 concentration is effective and cheap. Purpose-made exterior mould treatments (sold at Bunnings and most hardware stores in Darwin) work similarly and are often easier to apply evenly via a pump sprayer. Apply generously, making sure you saturate the growth rather than just wetting the surface.
Let it dwell for at least 15 minutes — longer for heavy infestations, up to 30 minutes. You should see the colour of the growth start to change from green/black to brown or white as the organism dies off.
Step 2: Pressure wash or scrub
Once the biocide has done its work, pressure wash at 2500–3000 PSI for concrete, or scrub with a stiff brush and rinse thoroughly for more delicate surfaces. Work from top to bottom so you're washing dead material down and off rather than back across clean areas.
Step 3: Seal or treat
On bare concrete or unsealed masonry, applying a penetrating sealer after the surface is fully dry significantly extends the time before regrowth. It's not a permanent fix in Darwin's climate — nothing is — but it can push your cleaning cycle from every 6 months to every 12–18 months. For exterior painted surfaces, a mould-inhibiting additive in your topcoat does the same job.
Internal Mould: A Different Beast
Internal mould in Darwin is almost always driven by condensation from air conditioning — cold surfaces meeting humid interior air — or by moisture penetration through walls, roofs, or window seals. In homes in Wanguri or Leanyer where ceiling insulation was installed decades ago and may have compressed or shifted, heat transfer and condensation issues are common contributors.
For surface mould on paint or render (the kind that wipes off but keeps coming back), the treatment is simple: wash with a bleach solution, let dry completely, then repaint with a mould-resistant interior paint. The painting step is non-negotiable — just cleaning and leaving bare surfaces gives the mould a perfect substrate to re-establish within weeks.
If you're finding mould recurring in the same spots despite cleaning and repainting, the cause is almost certainly moisture penetration or a ventilation problem rather than a cleaning problem. That's a different job — and often involves inspecting roof and gutter condition, checking window seals, or improving bathroom exhaust ventilation. Our guide to preventing humidity damage in Darwin homes covers the broader strategies for keeping moisture out of your walls and ceilings.
Rooflines, Eaves, and Gutters
Darwin homes with wide eaves — very common in tropical design — trap humidity under the overhang. Eave soffits, particularly fibrous cement or older board materials, are notorious for mould growth that's hard to treat without a ladder and some commitment. Timber fascias and soffits in original homes in Fannie Bay and Parap often have decades of paint layers, and mould can establish in cracks between those layers where no surface treatment reaches.
The connection to gutters matters here: gutters clogged with leaf litter stay wet between rain events, and that moisture wicks up into fascias and eave material. Keeping gutters clear is genuinely one of the best things you can do to slow mould growth on your eaves. Our article on gutter cleaning in Darwin's tropical climate covers why this matters more than most homeowners realise.
Algae on Roof Tiles and Metal Roofs
Green or black streaks running down roof tiles are algae, and they're extremely common in Darwin. On concrete tiles, they're cosmetic only — they don't damage the tile. On metal roofing, algae holds moisture against the surface and can accelerate corrosion, particularly in coastal areas where salt is already doing the same job. Low-pressure chemical washing (not high-pressure — that can force water under laps and into the roof cavity) is the right approach here.
Roof cleaning is specialist work. The risk of slipping on algae-coated tiles, combined with the height involved, makes this firmly a professional job rather than a DIY project.
When to Call AusMCo
AusMCo handles external mould and algae removal as part of full property cleans, including walls, concrete, eaves, fences, and driveways. For internal mould issues that keep recurring after surface treatment, the team can also identify the underlying moisture cause. See the pressure cleaning service page for exterior treatments, or our home repairs page for structural moisture issues.
Ready to stop fighting mould on your own? Contact AusMCo for a no-obligation quote on your property.