An unprecedented "thunderstorm asthma" event has left four people dead and three fighting for their lives in Australia, with authorities Thursday scrambling to review their procedures after emergency services were overwhelmed.

Some 8,500 people have presented to hospital emergency departments, mostly in Melbourne, since a thunderstorm coincided with a high pollen count on Monday, causing asthma and hay fever sufferers respiratory problems, Victorian state Health Minister Jill Hennessy said.

"When we've had people calling for ambulances -- one call every four-and-a-half seconds at the peak -- it was like having 150 bombs going off right across a particular part of metropolitan Melbourne," Hennessy told the Australian Broadcasting Corporation.

The four victims were aged between 18 and 35, reports said, with Victoria's health department reporting three others in a critical condition as a review got underway into how better to respond and manage such unusual phenomena.

"This was a health emergency of an unprecedented scale," Hennessy said, adding that those affected appeared to have suffered respiratory problems and cardiac conditions.

"Of the 8,500 people that were presenting to emergency departments on Monday and Tuesday, those with cardiac and chest pain featured very, very prominently."

The unusual phenomena is only known to have occurred in Australia several times, Asthma Foundation of Victoria chief Robin Ould said.

"When rye grass pollen becomes wet through humidity or water, it breaks up into a lot of small pieces and those small pieces can get past the nasal passage into the lungs. Normally rye grass would be trapped in the nasal passage," Ould told AFP.

"When it gets into the lungs, the allergens that are there cause an asthma attack... the small bronchial tubes become inflamed, they fill with mucus and the muscles around them become tight and people can't exchange their air."

Ould said those affected had asthma that is triggered by rye grass, or suffered from hayfever.

Melbourne was particularly susceptible to "thunderstorm asthma" events as rye grass was found predominantly in the parks and farmlands around the city, the capital of Victoria, he added.

About 10 percent of Australians have asthma -- in which the immune system in the airways goes into overdrive, and wheezing, coughing and restricted breathing result -- with 80 percent of them also experiencing hayfever, according to Asthma Australia.