Authorities are warning there is a high risk of thunderstorm asthma in northern Victoria on Sunday.
The Department of Health said the greatest risk was in the north central and northern country districts, which include regional centres Bendigo, Shepparton, Echuca and Castlemaine.
The warnings for all other districts were moderate.
Authorities issue thunderstorm asthma warnings when there is a high pollen count and a strong chance of thunderstorms.
The Bureau of Meteorology is forecasting storms across the state on Sunday.
The Department of Health's Angie Bone said those at risk should keep their inhaler on hand, and go inside if a storm arrived.
She said they should turn their air conditioners off and close their windows.
"Thunderstorm asthma can cause quite severe asthma in people who have hayfever or asthma, so it's important that we stay alert but not alarmed," Dr Bone said.
"It only affects people who are allergic to pollen."
The pollen count was extreme on Sunday in the south-west, Gippsland and Wimmera regions, according to the Melbourne Pollen Count and Forecast.
It was high in the central, Mallee and northern country districts, and moderate in north-central and north-east.
Experts said Victoria's pollen count was higher than usual this year in part because of the La Niña weather pattern, which results in conditions which draw pollen into the air.
November is usually the worst month for hayfever sufferers, with pollen counts beginning to drop off in December.
In 2016, Melbourne experienced the largest thunderstorm asthma event ever recorded, resulting in more than 3,500 emergency presentations, 35 ICU admissions and 10 deaths.