Bushfire smoke disrupted the Australian Open build-up on Wednesday for a second straight day to deepen concerns about the fate of the year's first tennis Grand Slam, but a cool change late in the day raised hopes of rain soaking the blazes.

The toxic haze that descended on Melbourne, where the Australian Open is due to begin next week, drifted down from out-of-control fires that have endured for months in eastern and southern Australia.

The bushfires, unprecedented in their duration and intensity, have claimed 28 lives while raising awareness about the type of disasters that scientists say the world will increasingly face due to global warming.

In Melbourne, a picturesque bayside city famed as one of the most liveable in the world, the bushfire smoke raised pollution levels to "hazardous" at the start of the week.

The bleak conditions continued on Wednesday, with residents donning face masks while dozens of flights were cancelled at Melbourne airport because of poor visibility.

Australian Open organisers pushed ahead with qualifying rounds on Tuesday.

But dramatic scenes of players dropping to their knees and choking, and one retiring due to the smoke, led to complaints about them being forced to stay out on the court.

With the air still tasting and smelling of smoke on Wednesday morning, organisers suspended qualifying rounds until 1:00pm (local time) on Wednesday.

Racing Victoria also cancelled two-horse race meetings on Wednesday.

With the pollution levels improving slightly, Australian Open organisers restarted play on Wednesday afternoon under better but still hazy conditions.

Rain hopes

Thundery weather then swept in late on Wednesday afternoon, bringing heavy rain that forced play to be cancelled for the day but raised expectations of clearer air for Thursday.

There were also hopes that the rain would extend to other parts of southern and eastern Australia where dozens of fires are still raging out of control and threatening to devastate many more rural towns.

Some bushfire and drought-hit areas could see 50-100 millimetres of rain, the Bureau of Meteorology said.

However, it said the "hit and miss" nature of thunderstorms meant it was difficult to predict exactly where the heaviest rain would fall.

The fires have already destroyed more than 2,000 homes and burnt 100,000 square kilometres of land — an area larger than South Korea or Portugal.

The confirmed death toll rose to 28 on Wednesday when authorities said they had confirmed a firefighter who died in late November in a traffic incident had at the time been trying to contain a blaze.

The fires have dominated headlines around the world and led to an international outpouring of aid for victims, as well as animals that have been injured in the blazes.

About one billion animals may have died in the fires and driven many species closer to extinction, according to environmental groups.

Australia's koala population has taken an "extraordinary hit" and could be listed as endangered for the first time, Environment Minister Sussan Ley has said.

Tennis disruptions

In Melbourne, the smoke has raised the prospect of interruptions and delays for the two-week Australian Open, which is due to begin on Monday.

Slovenian Dalila Jakupovic was forced to retire while leading in her qualifying match on Tuesday because of the smoke.

"I was really scared that I would collapse," she said.

Other players, including world number five Elina Svitolina, hit out at organisers for allowing qualifying to go ahead on Tuesday.

"Why do we need to wait for something bad to happen to do an action," she tweeted.

Climate change debate

Meanwhile, a debate about the attitudes of Australia's political and media establishment to climate change continued on Wednesday with a rare intervention from one of Rupert Murdoch's sons.

James Murdoch hit out at his father's media empire, which includes the News Corp group that dominates Australia's press landscape as well as Fox News in the US, for climate change "denial".

"They are particularly disappointed with the ongoing denial among the news outlets in Australia given obvious evidence to the contrary," said a statement from James and his wife, Kathryn.