A phenomenon called "thunderstorm asthma" has caused a huge spike in calls to Ambulance Victoria on Monday in the space of just a few hours.
A severe weather change at about 6pm brought with it thunderstoms, rain and strong winds that churned up pollen, dust and other irritants in the air.
Ambulance Victoria's State Health Commander Paul Holman said every available ambulance had been dispatched and more than 190 cases were waiting during the worst of it.
"I think a spike in calls would be the understatement of a century," he told AAP.
"Every ambulance and ambulance manager was recalled," he told The Age.
"It was an unprecedented night.
"I have never seen the computer dispatch of Melbourne look like it. It was what we call a sea of red."
Paramedics called in help from the MFB, police and doctors to help with the demand.
"These were quite sick patients," Mr Holman said, saying most jobs were code ones or top priority emergencies.
On a normal night, there would be about 10-20 cases waiting mostly of low or middle priority level.
Extra staff were put on the emergency triage phone service to make sure all patients were spoken to see if they needed an ambulance or could instead go to their local GP, Mr Holman said.
By 10:30pm, about 90 cases were still waiting, but they were of lower priority.
Mr Holman said it had been an "unprecedented phenomena".
"It's something in my 40 years I've never seen before," he said.
He expects most of the cases to be dealt with by midnight on Monday.
