A sixth person has died from an attack of thunderstorm asthma in Melbourne, and three more are still fighting for their lives in hospital.

The most recent victim, who has not been named, died on Saturday night in the city's eastern suburbs.

Five patients remain in intensive care in Melbourne hospitals. Three are considered critical.

Up to 8,500 people have been affected by ‘thunderstorm asthma.’A thunderstorm in Melbourne, the second most populous city in Australia, has been causing widespread respiratory issues among asthma and hay fever sufferers since Monday.

In all, 8,500 patients were treated on Monday and Tuesday in Melbourne, a city of 4.5 million residents, according to officials.

The demand for ambulances was so high that “it was like having 150 bombs going off right across a particular part of metropolitan Melbourne,” State of Victoria Health Minister Jill Hennessy said on Thursday.

Thunderstorm asthma attacks come after pollen particles multiply dramatically in thunderstorms, meteorologist David Brown says.

It's been one week since the unusual asthma attack outbreak overwhelmed the city's emergency and hospital services.

One of the victims in the outbreak a week ago had been waiting 30 minutes for an ambulance.

Twenty-year-old law student and life-long asthma sufferer Hope Carnevali was one of three people who died during a mass occurrence of thunderstorm asthma.

Officials say the weather event put more strain on the health system than a mass casualty terror attack.