The first public asthma warning has been issued after Melbourne suffered the worst-known thunderstorm-induced asthma epidemic in global history.

The warning comes as Health Minister Jill Hennessy admits the state was not prepared for a weather-triggered health disaster of the scale of last week's asthma crisis, which has claimed eight lives with one other person still fighting for their life more than a week later.

Victoria's Inspector-General for Emergency Management's interim report into how emergency services managed the "perfect storm" is now expected to be delivered in January, Ms Hennessy said.

Chief health officer Charles Guest issued a warning ahead of possible thunderstorms across Gippsland and parts of north-east Victoria on Wednesday afternoon.