Three people are still fighting for their lives after Monday night's sudden outbreak of "thunderstorm asthma", which saw 8500 people treated in hospitals across Melbourne.

Four people died after suffering asthma attacks in the unprecedented emergency that pushed Melbourne's hospital system to its limits, although it is feared more deaths may have not yet been attributed to the outbreak.

For the first time on Thursday – 72 hours after the freak weather event – the Department of Health confirmed that 8500 people had been treated in Melbourne hospitals on Monday and Tuesday.

Eight patients were still in intensive care at 5.30pm on Thursday, the department said.