Mould is one of the most common problems in New Zealand homes, especially in older houses, poorly ventilated bathrooms, and damp southern climates. Simply painting over mould won't fix the problem — it will keep growing underneath and push through your new paint within weeks. Here's how to treat it properly so your paint job actually lasts.
Why You Should Never Paint Straight Over Mould
Mould is a living organism. Paint — even mould-resistant paint — cannot kill established mould growth. If you paint over it, you're sealing in moisture and organic matter that the mould feeds on. Within a short time, you'll see dark patches bleeding through your fresh coat, and the surface may bubble or peel. Beyond the cosmetic damage, mould poses real health risks, particularly for people with asthma or respiratory conditions. BRANZ research consistently highlights moisture and mould as key contributors to poor indoor air quality in NZ homes.
Identify the Source of Moisture First
Before you touch a paintbrush, find out why mould is growing. Common causes include:
- Poor ventilation in bathrooms, kitchens, or laundries
- Condensation on cold walls and windows (very common in NZ winters)
- Leaking roofs, pipes, or gutters
- Rising damp from subfloor areas
If you don't fix the moisture source, mould will return no matter how well you treat and paint the surface. Check Energywise for practical advice on improving home ventilation and reducing condensation.
Safety First: Protect Yourself
Before you start cleaning mould, take precautions:
- Wear disposable gloves, safety glasses, and an P2 respirator mask (not just a dust mask)
- Open windows and doors to ventilate the space
- Keep children and pets out of the area
- Bag and dispose of any cloths or materials used to wipe up mould — don't rinse them in the sink
For large areas of mould (generally considered anything over roughly one square metre), or if mould has penetrated into wall cavities or insulation, consult WorkSafe NZ guidelines and consider engaging a professional remediation specialist.
How to Treat Mould Before Painting
Step 1 – Kill the Mould
The most widely used DIY treatment is a diluted bleach solution: mix one part household bleach with four parts water. Apply it to the affected area with a sponge or cloth, let it sit for at least 10–15 minutes, then wipe clean. This kills surface mould effectively on non-porous surfaces like painted walls and ceilings.
For a ready-made option, products like Resene Moss & Mould Killer (available from Resene ColorShops) are formulated specifically for this purpose and are a reliable choice before repainting. Dulux and Wattyl also offer pre-paint mould treatment products worth considering.
Step 2 – Clean the Surface Thoroughly
Once the mould is killed, wash the surface down with a sugar soap solution to remove any residue, grease, or dirt. Rinse with clean water and allow the surface to dry completely — this is critical. Painting over a damp surface is one of the most common causes of paint failure.
Step 3 – Sand and Repair if Needed
If the mould has caused the existing paint to bubble, flake, or peel, you'll need to scrape back loose paint, sand the edges smooth, and fill any damage with an appropriate filler. On GIB plasterboard surfaces, check whether the paper face has been compromised — badly damaged board may need to be replaced rather than repainted.
Step 4 – Apply a Mould-Inhibiting Primer
This step is often skipped but makes a big difference. A mould-inhibiting primer or sealer creates a barrier and gives your topcoat the best chance of staying mould-free. Resene's Waterborne Smooth Surface Sealer or a dedicated anti-mould primer are good options. Ask in-store at Mitre 10, Bunnings, or PlaceMakers for what's available in your area.
Step 5 – Use a Quality Mould-Resistant Topcoat
Finish with a mould-resistant paint designed for wet areas or high-humidity spaces. Most major NZ paint brands offer bathroom and kitchen-specific formulations with built-in mould inhibitors. In wet areas like showers and around baths, ensure your paint and surface preparation also comply with NZ Building Code clause E3 (internal moisture), which sets requirements for wet area linings and finishes.
Comparison: Treatment Options at a Glance
| Method | Best For | Notes |
|---|---|---|
| DIY bleach solution | Light surface mould on painted walls | Cheap, effective — ventilate well |
| Commercial mould killer spray | Moderate mould, convenience | Follow label instructions carefully |
| Mould-inhibiting primer | All situations before repainting | Don't skip this step |
| Mould-resistant topcoat paint | Bathrooms, kitchens, laundries | Choose products rated for wet areas |
| Professional remediation | Large or recurring mould, wall cavities | Required for serious infestations |
When to Call a Professional
Some mould situations are beyond a DIY fix. Call a qualified painter or remediation specialist if:
- Mould covers a large area or keeps coming back after treatment
- You suspect mould is growing inside wall cavities or under flooring
- There's a strong musty smell even after surface cleaning
- A household member has health issues aggravated by mould exposure
If the mould is linked to a plumbing leak, that repair must be carried out by a licensed plumber — this is restricted work under the Plumbers, Gasfitters and Drainlayers Board rules. Similarly, if fixing the moisture problem involves changes to your home's ventilation system involving electrical work, that must be done by a registered electrician under EWRB regulations.
For a professional painter who knows how to handle mould properly, search your local area on tradedirectory.co.nz to find qualified tradespeople near you.