The risk of thunderstorm asthma is "moderate" across much of Victoria with the rating expected to remain in place for much of the state on Friday.

A warning was issued on Thursday that there may be a high pollen count or a severe thunderstorm in certain areas, the Health Victoria website shows.

Regions at moderate risk include the Wimmera, South West, Central, West and South Gippsland, with people who may develop symptoms urged to avoid storms, have a reliever on hand and know their asthma action plan.

A moderate warning has also been put in place for Friday for the central, northern, northeast and Gippsland regions.

Authorities are on high alert to issue warnings to the community after 10 people died in Melbourne during a rare thunderstorm asthma event in November 2016.

Thunderstorm asthma happens when a large group of people experience asthma, often triggered by high pollen levels mixed with weather conditions.