Since 2023, our team has analysed more than 3,000 spore traps and 5,000 surface tape lifts, and carried out more than 1,000 site inspections across New Zealand and Australia. This article describes what we see, week after week, in the lab and on site, and where New Zealand buildings often differ from what overseas references would lead you to expect.
Hidden mould in modern New Zealand homes
A building's age is not a reliable guide to mould risk. We regularly find heavily contaminated homes built after 2010, while older homes of a similar size and location come up clean. The cause lies in design choices that have become standard over the past fifteen years: smaller eaves, box gutters with hidden downpipes, and flexible ducting for ventilation in roof spaces. Each of these can fail without anyone noticing. Rain backs up into ceiling spaces, or a flexible duct comes loose at a joint and condensation drips into the insulation. There are no signs outside, but inside the walls and roof space the classic water-damage moulds, such as Stachybotrys, Chaetomium and Ulocladium, can grow for months or years.
A Bio-Tape lift taken from a bedroom wall during an inspection. From a metre away the wall looked normal (how to take one yourself).
When there's no leak: the ventilation problem
More often, there is no leak at all, and the moisture comes from everyday living. Modern New Zealand homes are deliberately built airtight to hold heat, but without enough mechanical ventilation, warm indoor air meets cold outside walls and condensation forms on wall linings, windowsills and the backs of furniture. Aspergillus and Penicillium need very little moisture to grow on these surfaces. They can grow unseen and often dominate our air samples, while the people living there have on-and-off breathing symptoms and no visible mould anywhere in the home.
This is the most common reason New Zealand households ask us to test: not a wet patch or a stain, but unexplained sinus, asthma or tiredness symptoms that ease when they leave the house and return when they come back. Testing usually finds the cause. It is the most common hidden mould problem we see in modern New Zealand homes, and it comes directly from building homes to hold heat without ventilating them to remove moisture.
Sampling inside a cupboard where the wall had been damp for months. Cupboards, wardrobes and unventilated storage spaces are common places for hidden growth.
When framing is closed in wet: mould built into the walls
One pattern we see often enough to mention on its own is lapses in New Zealand's Dry Standard during construction. The Standard requires timber framing to be dried below a set moisture content before the building is closed in. Under cost and time pressure, weather protection is rushed or skipped, rain reaches the timber, mould starts to grow, and the home is sealed up around it. Months later the new owners move in, notice a mouldy smell or symptoms, and an inspection finds no visible growth on any surface, because the mould is inside the wall.
These are some of the hardest jobs we work on. Finding the problem needs specialist wall-cavity sampling tools, microscopy focused on water-damage moulds, and experience of having seen the pattern before. Repair costs can pass $100,000, and in the worst cases we have seen, repair wasn't economic and the home had to be rebuilt.
Measuring timber moisture during an inspection: we look for the cause, not just the mould. Checking framing moisture before the walls are closed in is by far the cheapest way to prevent this problem.
Older homes: a different pattern
Older New Zealand homes behave differently. They are draughtier: weatherboards move, sash windows leak air, and indoor temperatures swing with the weather. The mould we find in them is mostly Cladosporium, on window frames, around showers and on any surface that gets repeated condensation. That isn't risk-free, as long-term exposure to Cladosporium can irritate the airways, but it is usually a more visible, more localised and more treatable problem than the widespread Aspergillus and Penicillium growth we see in airtight modern homes.
How New Zealand compares with Australia
We have analysed samples from both New Zealand and Australia for several years, including leading the mould analysis for the NSW floods response in Australia, which covered more than 1,000 properties. With that much data from both countries, three differences stand out.
1. Aspergillus and Penicillium dominate New Zealand indoor air
In our New Zealand indoor air samples, Aspergillus and Penicillium together regularly make up more than half of all the fungal particles we identify. That is much higher than in our Australian samples, and it reflects New Zealand's mix of a damp climate, poor mechanical ventilation in modern homes, and outside walls prone to condensation. As a result, New Zealand households ask us about breathing problems and mould-related symptoms far more often than our overseas partner laboratories report. The pattern is consistent enough that raised Aspergillus and Penicillium in an indoor air sample, even at moderate counts, should be treated as a sign of indoor moisture problems, not just spores drifting in from outside.
An Aspergillus spore cluster in a New Zealand indoor air sample, analysed in our lab on the day this article was published. This amount is typical of what we find in modern airtight New Zealand homes.
2. New Zealand's outdoor spores are remarkably uniform, which helps
Australian spore profiles vary enormously by region, from cool-temperate Victoria to humid sub-tropical Queensland. Across our Australian work, outdoor reference samples can shift substantially between states and seasons. New Zealand, by contrast, is remarkably uniform: spore traps taken from the North Island and the South Island look essentially the same. We can't reliably tell an Auckland sample from a Christchurch one. That uniformity is useful. It means the comparison between indoor and outdoor samples, which every mould assessment relies on, is more reliable here than in Australia. When the indoor count or the indoor mix of moulds differs from the outdoor sample taken the same day, that points to mould growing indoors, not to regional differences.
3. Differences in particular moulds
Two differences come up again and again. Ulocladium is a stronger sign of water damage in Australian samples than in New Zealand ones: we find it more often, and in larger amounts, on water-damaged materials in Australia. Chrysosporium, on the other hand, turns up more often in our New Zealand samples, which we put down to the higher proportion of organic material (wool fibres, plant debris and animal-derived particles) in New Zealand household dust. We have also recorded Chrysosporium growing on the sarking paper of New Zealand homes after floods. That is unusual for a mould that normally prefers dry conditions, and it's the kind of observation that only comes from analysing thousands of samples from one country.
What this means for your property
The main lesson from the lab is that mould risk in New Zealand doesn't follow building age the way most owners expect. Modern airtight homes without enough ventilation can build up Aspergillus and Penicillium on surfaces prone to condensation, even with no visible water damage. New builds where the framing got wet can hide Stachybotrys and Chaetomium inside the walls from the day the keys are handed over. Older homes tend to have a different, more visible pattern, dominated by Cladosporium. The moulds named on your report tell you which of these situations you are dealing with, and whether the right response is better ventilation, an investigation inside the walls, or a simple surface clean.