The state's health authorities have been under mounting pressure, since the November 21 "thunderstorm asthma" epidemic saw the death of at least eight people and thousands of hospital admissions.

Despite the impending health disaster, no public warning was issued.

The Department of Health and Human Services' chief health officer, Ambulance Victoria's state health commander and the Bureau's regional director are meeting at the Bureau today and are expected to issue a warning to Victorians of another impending thunderstorm asthma event.

"Thunderstorms may develop across some parts of Victoria on Sunday particularly in western areas, with the passage of a cool change after a hot, humid day," a Bureau statement said.

"December is traditionally the most active month for thunderstorms in Victoria."

None of the organisations wanted to comment on what would be discussed, ahead of the meeting.

An online survey of 2000 people found about 40 per cent of those struck by thunderstorm asthma symptoms last week had not previously been diagnosed with asthma; and more than 90 per cent had experienced hay fever symptoms in the previous year.

Sunday's forecast in Melbourne will be warm and humid, 32 degrees with the chance of a thunderstorm later in the afternoon and evening.

The Inspector-General for Emergency Management is leading a state-wide review of the emergency response to the thunderstorm asthma event.

On Wednesday, Health Minister Jill Hennessy admitted the state was not prepared for a weather-triggered health disaster.

"This was a health emergency of an unprecedented scale, and we have an obligation to ensure that we learn every lesson there is to learn from this event," she said.

Ms Hennessy said an interim report into how the epidemic was managed, would be delivered in January.

On Wednesday morning, chief health officer Charles Guest issued a health warning for Gippsland and north-east Victoria, where thunderstorms had been forecast. This alert was later cancelled.