The pollen count in Canberra reached the highest ever recorded on Sunday, as health authorities have warned of a possible thunderstorm asthma event on Monday.
The Australian National University's Canberra pollen count found Sunday's extreme level at 193 pollen grains per cubic metre was the highest ever recorded in the ACT.
The count will remain high until Wednesday and increase to extreme levels from Thursday to Sunday.
ACT Health issued an alert for possible thunderstorm asthma events on Sunday and Monday.
A storm lashed the ACT on Sunday night dumping almost 30mm at Canberra Airport, according to the Bureau of Meterology.
Storms are predicted to hit again on Monday morning and afternoon with up to 15mm possible.
The ACT Health Directorate warned Canberrans with asthma and hayfever to be cautious as the wet weather hit.
A thunderstorm asthma event can happen when high grass pollen levels combine with strong winds from thunderstorms and can cause severe asthmatic reactions from some people.
"Not all thunderstorms produce an asthma event, and thunderstorm asthma is a rare event. However, people who suffer from hay fever and/or asthma are encouraged to be vigilant over the next two days for any worsening of their condition," ACT Health said in a statement.
"Where possible, avoid being outside during a thunderstorm - especially in the wind gusts that come before the storm. Go inside and close your doors and windows. If you have your air-conditioning on, turn it onto recirculate."
The weather will begin to clear on Tuesday when a partly cloudy top of 24 is expected.
Warm and sunny weather will return by the weekend with tops of 31 and 34 forecast on Saturday and Sunday respectively.