Warm weather and heavy rain has led to an extreme rating for the fungal spore known as alternaria being recorded by Canberra Pollen Count in recent days.

Alternaria spores are most commonly detected from late spring and into early autumn and can be found on the leaves and stems of plants.

Pollen tracker and Australian National University researcher Professor Simon Haberle said La Nina weather conditions had made it a perfect breeding ground for the fungal spore in Canberra.

"These spores mostly occur when you have a warm and a wet climate, and the spores are produced when the fungus grows," Professor Haberle said.

"Once you have a couple of days' worth of rain, it takes around two to four days and you see a spike or peak in the spores, so there is a bit of a lag period."

Mowing lawns and harvesting crops often result in releasing the spores into the air.

Extreme levels of alternaria spores have been experienced in Canberra four times since mid-October last year.

Professor Haberle said those extreme levels were set to be even more prevalent in coming weeks as the storm season continues.

"This has been quite a significant season for alternaria, and it's been for the same reason we had a record pollen season [during spring] with high pollen counts," he said.

"It has continued into the mid-to-late summer, which is a period where we normally don't see particles in the air."

The data comes as 2020 marked one of the worst pollen seasons in Canberra in recent years.

In the past 12 months, the national capital recorded seven days of an extreme pollen count and 24 days of a high pollen count.

In an average year, there is just one extreme day and four days of a high pollen count, with the previous high of three extreme days in 2014.

Professor Haberle said high pollen season normally runs from October to December in the ACT, which is normally driven by grass pollen levels. However, the alternaria spore levels are expected to continue through until autumn.