Six people are dead after suffering from a rare illness known as thunderstorm asthma in Australia.

Five patients remained in intensive care units in Melbourne and three of them were in critical condition, a health department statement said.

Thunderstorm asthma occurs when extreme winds and air moisture break pollen particles of rye grass up into small enough pieces to enter peoples lungs.

The storm overwhelmed emergency services and hospitals in the city of 4.5 million people, with 8,500 receiving hospital treatment.

Danny Hill, acting general secretary of Ambulance Employees Australia, said it was the "busiest night" many paramedics had worked in their careers.

He said: "There was about 30 people who were so severely ill that they had to be taken straight into intensive care. So you're talking an event equivalent to a bushfire where people are severely burned, or a terrorist attack where people are critically injured.

"That's the level of chaos our crews dealt with on Monday night."

A major investigation into the response of emergency services to the event has been ordered.