Severe thunderstorms bringing heavy rain and hail to parts of Victoria have started to improve air quality in Melbourne, after "hazardous" ratings across much of the state prompted concerns about the long-term health impacts of bushfire smoke.

Key points: Up to 44mm of rain fell in 30 minutes as storms swept through Geelong

Up to 44mm of rain fell in 30 minutes as storms swept through Geelong Melbourne's air quality improved to "good" after being "hazardous" earlier

Melbourne's air quality improved to "good" after being "hazardous" earlier Smoky conditions saw more than 50 domestic flights cancelled at Melbourne airport

The Environment Protection Authority's AirWatch website rated air quality as "hazardous" in Melbourne, Geelong, East Gippsland, Bright and Wangaratta for much of Wednesday as smoke from fires in Victoria's east and north-east persisted across the state.

As storms moved across the state late on Wednesday afternoon, air quality across Melbourne started to improve to "moderate" levels and by evening to "good".

Severe thunderstorms hit Geelong in the early afternoon, bringing heavy rain and hail to some suburbs, but it did little to get rid of the persistent smoke haze, with air quality only dropping one level to remain "very poor".

Avalon Airport had 44mm of rain in just 30 minutes and recorded winds of 113 kilometres per hour.

The Bureau of Meteorology (BOM) issued a severe thunderstorm warning for much of central Victoria, including Melbourne during the afternoon peak hour.

It warned the storms could bring intense rainfall in "one of the biggest thunderstorm threats this season", which could cause flash flooding and mudslides.

By evening, the threat of severe storms over Melbourne had eased, with the slow-moving rain band moving into the east of the state.

The weather bureau issued a severe thunderstorm warning for Melbourne and surrounding suburbs. ( ABC News: Iskhandar Razak )

Meghan said the storm came through Bendigo at around 4:00pm.

"There was flash flooding and so much thunder I couldn't hear the radio in the car," she told ABC Radio Melbourne.

Peter was driving outside Ballan, west of Melbourne and said the rain was so heavy "I couldn't see".

"The rain was ferocious, I've never seen anything like it before," he told ABC Radio Melbourne.

"For 10 minutes there were people going everywhere. They couldn't see. A lot of people pulled up."

After hitting a top of 36C the temperature plunged almost 15 degrees. ( ABC News: Darryl Torpy )

There were also reports of heavy thunder and lightning.

"I've not in a long time seen rain quite like this," talkback caller Gillian said.

"It's just come down in one great big downpour and the worst part is there are quite a lot of places where the water is across at least one lane of the road.

"People really need to slow down."

Heavy rain swept across Melbourne, improving the air quality. ( ABC News: Darryl Torpy )

The smoke pollution over the past two days has caused major disruptions across Melbourne and Geelong, as beaches and pools have been closed and sporting events including horseraces have been cancelled.

More than 50 domestic flights were cancelled at Melbourne airport, and spokesman Grant Smith said there were also "too many to count" delays due to the conditions.

Further cancellations were expected due to the severe storms, and passengers were urged to contact their airline to see if their flights would be affected.

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Ambulance callouts rise

Victoria's chief health officer, Brett Sutton, said paramedics had responded to more than 100 callouts for people struggling to breathe on Wednesday morning.

"I think we can expect a doubling on days like today and yesterday in terms of ambulance callouts," he said.

The conditions caused concern for Australian Open tennis players on Tuesday, with Slovenian player Dalila Jakupovic abandoning her qualifier mid-match after falling to her knees in a coughing fit.

"I just couldn't breathe anymore and I just fell on the floor," Jakupovic said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 24 seconds 1 m 24 s Dalila Jakupovic said she was "really scared" she would collapse during the qualifying match.

Dr Sutton has urged Tennis Australia to adopt an air quality policy, warning that Victoria was likely to see similar smoke pollution in the future.

"We have experienced the worst air quality in Melbourne for a long time in terms of protracted days and hazardous levels but it might be a new reality," he said.

"It will be more probable, we know that going forward in terms of the bushfire season, how bad it might be and how long it might be."

Dr Sutton said the Department of Education was already developing an air quality policy as it prepared for students to return to school.

One pollution website has ranked Wangaratta's air quality as the third-worst in the world on Wednesday. ( Twitter: @AusCelticsFan )

Melbourne confronts 'surreal' new reality

Outside Melbourne Park, Melburnians told the ABC they were surprised by the volume of smoke lingering across the city.

Juliana Pedrosa, who was dropping her 17-month-old daughter Athena off with family before she went to work, said she was concerned about the impact of the smoke on her daughter.

"I wouldn't take her out of the house but I have to work and my mother-in-law is going to pick her up here," she said.

Juliana Pedrosa said she had never seen air pollution like it during her 10 years in Melbourne. ( ABC News: Stephanie Ferrier )

Lachlan Hamilton said he now lived in the United Kingdom but was back visiting family and the smoke seemed "unreal".

"It's pretty surreal being back in Melbourne with the haze around," he said.

Smoke haze that has hung over Melbourne this week is likely to return on the weekend. ( Supplied: Alex McLennan )

The EPA's state agency commander Steve Lansdell said the smoke would affect Victoria for months and areas near the fires would be worst affected.

"That's obviously where some of the biggest risks and concerns are, with the extended smoke impact there and the intensity of it," he said.

Health fears for people in worst-affected smoke zones

Dr Sutton said the smoke impact was greatest in bushfire-affected areas where people could not easily relocate, and were exposed to consecutive days of poor air quality.

"They're in shelters and incident control centres and help centres where they have basically got indoor air quality that's as bad as Melbourne's was yesterday," he said.

Smoke pollution has persisted over Gippsland for weeks. ( ABC News: Mark Debono )

The bushfires producing the smoke have destroyed or damaged more than 368 homes and last night threatened lives again in East Gippsland.

Victorian bushfire facts and figures 340 homes lost

340 homes lost 532 sheds or other structures damaged or destroyed

532 sheds or other structures damaged or destroyed 19 ongoing fires

19 ongoing fires More than 1.4 million hectares burnt

More than 1.4 million hectares burnt 1,500 firefighters

1,500 firefighters 100 international firefighters working in East Gippsland and Hume

100 international firefighters working in East Gippsland and Hume 140 more international firefighting personnel arriving late January

The fire that devastated Mallacoota at the start of the year flared and moved west towards the small community of Tamboon, prompting an emergency warning for the area.

About 14 people remained in the community as the fire moved across the area, but no-one was injured and no properties were lost.

"We know a number of those were sheltering on the beach late last night and early this morning," Victoria's state response controller Gavin Freeman said.

Prime Minister Scott Morrison said on Sunday he would take a proposal to establish a royal commission into the bushfire crisis to Cabinet.

Peter Marshall, the national secretary of the United Firefighters Union, said on Wednesday a royal commission was unnecessary because there were already plenty of recommendations from previous inquiries, including the one into the 2009 Black Saturday disaster.