Bushfires rage as heatwave grips Australia's southeast

An out-of-control bushfire has shrouded Melbourne in smoke as much of Australia remains in the grips of a summer heatwave.

With hot temperatures persisting across inland parts of Australia's southeast, six major bushfires were burning in Victoria on Sunday.

The Carlisle River fire near the Otway Ranges sparked an emergency warning after breaking containment lines about 6pm on Saturday.

The warning level was downgraded to watch and act on Sunday afternoon, although the fire was yet to be brought under control.

The blaze grew to about 8500 hectares overnight and drew within a kilometre of Gellibrand, threatening homes in the rural town about 175km southwest of Melbourne.

"It did actually burn back into some of the areas that have been previously burned," Forest Fire Management Victoria's Chris Hardman told reporters on Sunday.

"But it's a long way from being contained."

Rapid assessment teams were sent to determine if any buildings or homes on the town's outskirts had been lost.

"Certainly we could see some losses as a result of the fire activity," Mr Hardman said.

There was no immediate threat to the popular tourist spots of Lorne and Apollo Bay, but authorities warned that could change.

Winds sent smoke hovering over Melbourne, Geelong, the Bellarine Peninsula, Ballarat and the Otways, leading authorities to issue a warning for poor air quality.

Melbourne had the sixth-worst air quality in the world as of about midday on Sunday, according to the IQAir website.

Bureau of Meteorology forecaster Diana Eadie said the smoke would linger for certain communities but it had largely cleared in Melbourne by mid-afternoon.

Tarcoola in central South Australia recorded the country's highest temperature of 48.2C on Saturday.

Conditions eased across southern parts of Victoria but hot temperatures remain in the north, where total fire bans are in place for four districts.

The mercury in Hopetoun and Ouyen, in Victoria's northwest, is forecast to reach 49C on Tuesday, potentially breaking the statewide temperature record of 48.8C set on Black Saturday in 2009.

Temperatures are predicted peak in the mid to high 40s on Tuesday across swathes of western NSW, southwest Queensland and central and southeast South Australia.

Adelaide has a forecast top of 45C on Monday, Melbourne is tipped to hit a high of 43C on Tuesday and Canberra is bracing for 42C on Wednesday.

Several locations in Victoria's north are facing the prospect of seven consecutive days of maximums above 40C.

"We haven't seen heatwave conditions like this in Victoria for almost 20 years," Emergency Management Commissioner Tim Wiebusch said.

People were encouraged to take preventative steps to avoid heat stroke, which had a high fatality rate, Ambulance Victoria's Dale Armstrong said.

Victorian paramedics were called to rescue 11 children left in hot cars on Saturday.

With a cool change failing to filter through to the state's northeast, firefighters were struggling to keep the Walwa fire on the Victorian-NSW border at bay.

Mr Hardman said the fire, which had scorched more than 100,000 hectares, could break containment lines as early as Sunday afternoon or Monday.

Several bushfires in South Australia and Western Australia were also burning out of control on Sunday.

A grassfire was burning uncontrolled at Keilira, in the state's southeast.

Locals were urged to leave before crews slowed the spread of the fire, which has burned 50 hectares.

In WA, a watch and act warning was also in place for a fire burning in the Fitzgerald River National Park, about 500km southeast of Perth.

Authorities said there was a threat to lives and homes and anyone who had already left the area was advised it was not safe to return.