The Canberra Pollen Count and Forecast service says the nation's capital recorded seven extreme and 24 high pollen count days in 2020.

An extreme pollen day is when the average daily concentration of grass pollen is 100 grains or more in a cubic metre of air. A high day is when there is 50 grains or more.

In an average year Canberra normally experiences just one day where the pollen count is at an extreme level and four days at a high level. The previous highest number of extreme days in a single year was three in 2014.

The high amount of pollen was due to weather conditions, according to the Canberra Pollen website.

"The weather over the last six months played a key role in producing conditions suitable for the bumper grass pollen season of 2020," the website said.

"Elevated rainfall related to La Nina conditions in the Pacific Ocean and the persistent rains of spring and early summer around Canberra resulted in high grassland productivity and a greater potential for grass pollen to be produced."

In the years prior to 2020, grass pollen levels remained relatively low in the territory due to dry conditions. According to Canberra Pollen, in 2017 there were only two moderate days and no high or extreme pollen count days.

A moderate day is where 20 grains or more in a cubic square metre.

The service takes into account pollen count data from 2007 to 2009 and 2014 to 2020.

Canberra Pollen Count and Forecast says a significant feature of the season was the five thunderstorm asthma warnings that were issued for the ACT.

It is the first year the warnings have been issued in the territory.

The high pollen season in Canberra usually runs from October to December.