The Australian mould season calendar: when mould is worst by city and month

In the tropical north, the mould season follows the monsoon. The Bureau of Meteorology defines the northern wet season as roughly November to April, running October to April in the far-north Northern Territory, with the monsoon onset usually arriving near Darwin in late December. Through those months the air stays saturated and rain is close to daily.

Darwin is the clearest case. Its mean 9am relative humidity peaks at 83% in February, and January is its wettest month at 432.1 mm, part of a 1,736.8 mm annual total spread across about 113 rain days, almost all of it in the wet season. Cairns is wetter still, the highest annual rainfall of the six cities at 2,025.5 mm, with February its wettest month at 437.5 mm and morning humidity holding at 78% from February through April. Brisbane sits between the two. Its rain peaks in February at 155.1 mm, yet its morning humidity is actually highest in winter, at 70% in June, a reminder that even the subtropics get a cool-season damp lift.

Down south the driver is not rain, it is condensation. Cold outdoor air holds less moisture, but inside a heated, closed-up winter home that moisture collects on the coldest surfaces: single-glazed windows, uninsulated external walls, the backs of wardrobes. That surface dampness is what feeds mould, which is why the southern peak lands in the middle of winter rather than the wettest month.

Melbourne's mean 9am relative humidity peaks in winter at 83% in June and 81% in July, against a summer low of 64% in December, and its most frequent-rain month is August, with rain on an average of 15.4 days. Hobart follows the same shape, reaching 80% morning humidity in both June and July versus 60 to 61% in summer. Sydney is the in-between city. It has no dry season at all, its wettest month is June at 133.1 mm, not summer, and its morning humidity holds a steady 71 to 74% from February right through to June. That autumn-into-winter plateau, with no dry month to reset it, is Sydney's quiet mould window.

The building-science mechanism is the same everywhere. As the World Health Organization's 2009 indoor-air guidance puts it, "Excess moisture on almost all indoor materials leads to growth of microbes, such as mould, fungi and bacteria." Warm wet air in a Darwin February and cold condensation on a Melbourne July window are two routes to the same thing: moisture sitting on a surface long enough for mould to take hold.

Building-science consensus puts the relative humidity that favours surface mould activity at roughly 70 to 80%. That is an elevated-risk band, not a switch. Humidity in that range makes mould more likely, it does not guarantee it, and ventilation, surface temperature and how long the damp lingers all matter. It is worth reading the table with that band in mind. Darwin at 83% in February, Melbourne at 83% in June and Hobart at 80% in July are all sitting in or above it, just in different seasons.

The Bureau's climate data tells you when mould risk is highest. Two industry studies have tried to rank where it is highest. In Airtasker's 2025 mould index, built from its own job-request data rather than government figures, Sydney's Eastern Suburbs took the top spot as Australia's mouldiest area, scoring 61.32 out of 100 with mould-removal demand 547% above the national average, and Cairns ranked second at 60.67 on the strength of its warm, humid air and heavy rain (Airtasker). The same analysis found mould-related job requests grew 30.7% between 2020 and 2024, with a demand spike in July 2022. Separately, Compare the Market's search analysis found Cairns residents look up "damp issues" about 7,920 times a month, well ahead of Darwin and Canberra. Treat both as commercial studies, not official measurements. They track demand and search behaviour, not mould itself.

Mould is common. About 35% of the 22,550 households in the 2022 Australian Housing Conditions Dataset reported mould, the second most common building problem after wall cracks (AHURI, from Baker et al. 2022). That is self-reported, households saying they have seen it rather than a lab measurement, but it tells you this is a mainstream problem, not a fringe one.

The practical use of a calendar is timing. If you live in the north, the work is done before and during the wet season: get airflow and drying habits in place from October, not in February when the walls are already damp. If you live in the south, it is a winter job: run exhaust fans, open up on the dry days you do get, and keep an eye on cold walls and window reveals from June to August. Sydney is the odd one out, with no off-season, so it rewards steady habits year round rather than one seasonal push. Leaving damp to run is also the expensive option. Water-related defects, the main cause of mould, cost on average about A$7,000 more to repair than other building faults, from a Victoria University analysis of insurance claims (The Conversation). Catching the season early is cheaper than catching the repair late.

The six-city calendar is built from the Bureau of Meteorology's climate statistics for each city's long-record station: Darwin Airport (station 014015), Cairns Aero (031011), Brisbane Aero (040842), Sydney Observatory Hill (066062), Melbourne Airport (086282) and Hobart Airport (094008). Humidity figures are mean 9am relative humidity, except where the 3pm reading is named. Rainfall figures are mean monthly totals and rain-day counts are mean rain days. Peak mould-risk season is read from the combination of high humidity and high rainfall, not calculated as a single score.

A few honest caveats. This is a map of climate exposure, not a measurement of mould in any one home. Local factors, ventilation, building age, insulation, leaks and how a room is used, matter as much as the weather outside, which is why two homes in the same suburb can differ. The 70 to 80% humidity band is a general building-science guide, not a threshold that mould must cross. The Airtasker and Compare the Market figures are commercial studies of demand and search interest, not government data, and are labelled as such. The 35% prevalence figure is self-reported survey data. Bureau humidity averages are drawn from long-record climate tables and station record lengths vary.

Sources: Bureau of Meteorology climate statistics for Darwin, Cairns, Brisbane, Sydney, Melbourne and Hobart; World Health Organization, WHO Guidelines for Indoor Air Quality: Dampness and Mould (2009); Airtasker, Australia's mouldiest cities (2025); Compare the Market search analysis; the 2022 Australian Housing Conditions Dataset via AHURI and Baker et al. (Scientific Data); Victoria University water-damage analysis via The Conversation.

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Free to cite with attribution to the Aurala Naturals Australian Mould Season Calendar, 2026. Aurala Naturals makes First Light, a natural mould-defence blend made in Australia.