Two people have died after experiencing respiratory issues during a "thunderstorm asthma" emergency in Melbourne last night, Ambulance Victoria says.

Key points: Hospital staff say "they've never seen so many people" in emergency with same condition

Hospital staff say "they've never seen so many people" in emergency with same condition People allergic to rye grass particularly susceptible to thunderstorm asthma

People allergic to rye grass particularly susceptible to thunderstorm asthma Hundreds of calls for building damage, mainly in Melbourne's west

A severe thunderstorm swept through Melbourne yesterday after the state's hottest day since March, damaging a number of buildings, felling trees and spreading pollen.

Hospitals were swamped with emergency patients, while firefighters and police were called on to help paramedics respond to thousands of calls after the conditions caused breathing problems for Victorians.

Ambulance Victoria emergency operations general manager Mick Stephenson said two people died in Melbourne's western suburbs after reporting respiratory symptoms.

"We do know of those two cases and there will be other people who died in Victoria yesterday as well," he said.

"Whether or not they're related to this phenomena, we don't yet know. We'll have to review each incident and, as I say, in the fullness of time we'll understand whether or not they are related."

Mr Stephenson said one of the patients had waited at least 30 minutes for an ambulance.

An internal review was underway into the circumstance of the two deaths, he added.

There were 2,000 calls to triple-0 for ambulances between 6:00pm and 11:00pm — nearly seven times more than usual.

'We ran out of Ventolin puffers at one point'

St Vincent's Hospital, in the inner-Melbourne suburb of Fitzroy, was overflowing with emergency patients and ran out of Ventolin puffers.

"The emergency physicians say not only have they never seen anything like that before, they've never seen so many people arrive at one time all suffering the same condition," hospital spokeswoman Kathy Bowlen said.

"We were considering opening a second emergency department … we ended up doing that using the day procedures area.

"We ran out of Ventolin puffers at one point."

The hospital enacted its Code Brown plan to free up beds for extremely sicks patients during external emergencies.

Andrew Walby, the hospital's director of emergency medicines, said it was the first time, to his knowledge, a Code Brown had been called at the hospital in at least five years.

"We had one patient who was unable to manage to maintain their own breathing and ended up requiring to be put on a ventilator and go up to our intensive care unit," he said.

Health Minister Jill Hennessy said there would be a review into how hospitals and the ambulance service handled the emergency.

"No doubt there will be lessons to learn — there always are in these cases," she said.

"But every part of our emergency management surge [plan] occurred correctly."



'Extraordinary and unpredictable increase in demand'

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Sixty ambulance crews were called back into work and managers went out on the road, Mr Stephenson said.

"We essentially had a day's workload within five hours," Mr Stephenson said.

"In the 15 minutes from 7:00pm when we would expect about 30 triple-0 calls for ambulance there were 200 calls — that's a call every 4.5 seconds.

"This was an extraordinary and unpredictable increase in demand in a very short period of time."

Asthma Victoria chief executive Robin Auld said people allergic to rye grass were particularly susceptible to "thunderstorm asthma".

"Particularly, in this season, if you've got allergic reaction, keep up your preventer medication [and] talk to your doctor about the best way to use your medication," he said.

"Also, if you start to have symptoms, that's where your blue puffer comes into play."

More than 400 children were treated at the Royal Children's Hospital, and more than 60 patients were still waiting to be seen at 8:30am.

"The most seriously injured or sickest children are always seen first at [the emergency department] so children with less urgent conditions currently face long wait times to see a doctor," a statement on the hospital's Facebook page read.

Yarraville's Carnovale Pharmacy — the only 24-hour pharmacy in the inner west — was extremely busy overnight and ran out of asthma medication.

Storm clouds of Point Cook in Melbourne's west. ( ABC News: Kala Lampard )

Hundreds of calls for building damage

The State Emergency Service (SES) received 430 calls for building damage.

Most were from Melbourne's west, but one family had to be relocated after a tree crashed through their house at Dunolly in central Victoria.

Altona Meadows resident Marie Clement has lost all the tiles from a section of her roof.

"The noise was unbelievable, it was truly unbelievable," he said.