A sixth person has died in a mass asthma outbreak caused by a thunderstorm in Melbourne.

Five people remain on life support with three in a critical condition a week after heavy winds and rain hit Australia’s second-largest city.

The extreme weather on Monday night spread tiny pollen particles across the city, triggering a rare condition known as thunderstorm asthma.

Ryegrass pollen grains became soaked and broke open in the storm, dispersing very fine, highly allergenic particles, which can penetrate deep into the lungs.

The sixth victim died in in hospital on Saturday night from medical complications relating to the storm, according to the health department.

It said another 12 patients were still in hospitals with less serious respiratory and related conditions.

Around 8,500 people received hospital treatment in total, with a third of patients who suffered asthma attacks on Monday night reporting never having had asthma before.

In pictures: Storm Imogen hits UK Show all 15 1 /15 In pictures: Storm Imogen hits UK In pictures: Storm Imogen hits UK A man leans into the wind on the beach at Newhaven. Winter storm Imogen has brought rough seas and gale force winds to many coastal areas of Britain Reuters In pictures: Storm Imogen hits UK A man moves a traffic cone to close Fistral Beach car park because of strong winds in Newquay Getty Images In pictures: Storm Imogen hits UK Recovery vehicles work to right an overturned lorry on the M4 between Bridgend and Port Talbot as winds of nearly 100mph battered Britain after Storm Imogen slammed into the south coast bringing fierce gusts and torrential downpours PA In pictures: Storm Imogen hits UK Waves crash over the sea wall at Porthcawl in Wales PA In pictures: Storm Imogen hits UK A flooded road near Lower Brockhampton in Dorset PA In pictures: Storm Imogen hits UK Vehicles drive down a a flooded road near Lower Brockhampton in Dorset PA In pictures: Storm Imogen hits UK Fallen sixty foot oak tree blocks a road in the Hampshire village of Hook Rex Features In pictures: Storm Imogen hits UK Bruno, a bearded collie cross, walks in the wind in Newquay in Cornwall Getty Images In pictures: Storm Imogen hits UK A car which crashed into a fallen tree on the Romsey Road in New Forest, Hampshire PA In pictures: Storm Imogen hits UK A lorry lies overturned off the road when gale force winds hit Brighton EPA In pictures: Storm Imogen hits UK A collapsed wall in the Worcestershire village of Bretforton near Evesham which injured two children PA In pictures: Storm Imogen hits UK Waves hit a harbour wall in Newhaven, East Sussex Getty Images In pictures: Storm Imogen hits UK The P&O Pride of Burgundy is battered by waves as she arrives at the Port of Dover in Kent PA In pictures: Storm Imogen hits UK Waves hit a lighthouse on the harbour in Newhaven, East Sussex Getty Images In pictures: Storm Imogen hits UK A house in Hampshire where a crew from Gosport Fire Station were called to help as a trampoline got stuck on the roof of a conservatory following high winds PA

Local emergency services said they took six times more calls than the daily average between 6pm and 11pm local time.

“It was extraordinarily busy, it was unprecedented,” Mick Stephenson, Ambulance Victoria's executive director of emergency operations, told the BBC.

He said about 200 calls were directly linked to asthma, but 600 more reported respiratory issues.

“What we do know is that a lot of people who called last night had never had asthma before, so this was their first experience,” he said.

The world's first recorded thunderstorm asthma event occurred in Melbourne in 1987, when hospitals reported a five-fold increase in asthma cases.

Similar events have taken place in the United States, Canada, the UK and Italy. The last major occurrence in Melbourne was in November 2010.

Before this week, the largest known outbreak was in London in June 1994, when hospitals were inundated by 640 patients seeking emergency treatment for asthma and other breathing problems.

The Australian health minister Jill Hennessy said an investigation would be launched by the Inspector General for Emergency Management.

But the shadow health minister Mary Wooldridge said there had “clearly been a communications failure” on Monday night, according to the Herald Sun.

“This was life and death and there were no alerts to the media, the information was not going out to people,” she said.

The incident occurred on one of Australia's hottest November days. At 8am the temperature had already hit 26 degrees, rising to 35 degrees in the afternoon before the storm hit.