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.

Three were in a critical condition, two of them in intensive care at the Northern Hospital in Epping.

Sunshine Hospital was one of those under the greatest strain, with 18 ambulances banked up in the early hours of Tuesday morning as crews frantically worked on patients.

Victoria's Health Minister Jill Hennessy said overwhelmed emergency services weren't quite sure what they were dealing with on Monday night as the demand for help soared.

"When we have one bomb go off, we know what we're dealing with," she said.

"When we've had people calling for ambulances – one call every 4½ seconds at the peak – it was like having 150 bombs going off right across a particular part of metropolitan Melbourne.

"And that's something we've never really planned for."

One of the two critical patients at Northern Hospital is Broadmeadows dentist and grandfather Munawar Hussain, who suffered a cardiac arrest.

After desperately calling for an ambulance and being told there would be a lengthy wait, Mr Hussain's daughter drove him to the Northern Hospital at 8.45pm.

Nurses and doctors rushed out of the hospital to meet Dr Hussain at the main entrance of the Emergency Department. They performed CPR on the grandfather in the hospital's car park.

Among the four people whose deaths have been linked to the event were 20-year-old law student Hope Carnevali and Greenvale high-school student Omar Moujalled.

Ms Carnevali died in her family's arms on their front lawn after waiting more than 30 minutes for an ambulance.

Noble Park father-of-two, Clarence Leo, and 35-year-old man, Apollo Papadopoulos, died after suffering asthma attacks, also believed to be due to the weather event.

Thunderstorm asthma occurs when there is a sudden change in weather conditions.

Hot and dry winds drove Melbourne's temperature to 35 degrees on Monday, before a thunderstorm hit shortly after 6pm.

The storm and heavy rain caused rye grass pollen to absorb moisture and burst, dispersing smaller particles that became trapped in people's lungs.

The incredible "pollen load" led to the huge wave of people suffering breathing issues and cardiac arrest.

The demand for help was so great that Ambulance Victoria ran out of ambulances and had to call in police officers, firefighters, non-emergency patient vehicles and field doctors trained for disasters to help with transporting acutely ill patients to hospital.

Ms Hennessy, who has commissioned the Inspector-General for Emergency Management to review the emergency response, said the thunderstorm asthma event could not have been predicted, but that work was under way to create new models to predict the effect of extreme weather on health.

"Unpredictable weather patterns and the impact on health I think is a new emerging frontier for public health risks," she said.

Ed Newbigin​ from Melbourne University, who runs Melbourne's main pollen counting, station thinks it is possible to develop an asthma thunderstorm forecast thanks to data that exists from previous events.

He said Melbourne received conditions with high grass pollen and thunderstorms quite regularly in the spring and summer months and if the missing ingredient could be found, a forecast model could be created.

Government agencies, pollen scientists and the weather bureau were meeting on Thursday to discuss Monday's event.

With Henrietta Cook