One person died and officials warned that more lives could be lost as major bushfires raged across southeastern Australia Friday in heatwave conditions, with residents urged to heed warnings to "leave and live".

Police said the body was found in a house at Roses Gap in the the Grampians National Park west of Melbourne, currently sweltering through a major heatwave this week.

"Sadly, we've lost one person at Roses Gap. Police located the body of a person earlier this morning... in a house," said police commissioner Ken Lay.

"The death appears to be fire related."

The Northern Grampians fire, which is burning out of control over about 21,000 hectares and has prompted more than 100 people to evacuate the area, was running so hot it was "creating its own weather", said Victoria fire chief Craig Lapsley.

The convection column -- a plume of hot gases, smoke, ash and other debris created by the fire -- was causing lightning strikes which were starting other blazes, Lapsley said.

"These fires will be very intense and erratic this afternoon," Lapsley told reporters.

"There is a fair chance of losing property and even, if people are caught in the wrong space, life could be lost."

Lapsley said Friday was just the start of "what is a significant fire period" for the state, where a devastating firestorm in 2009 killed 173 people and destroyed more than 2,000 homes.

"Today is one of those days that certainly will be marked in the history of Victoria," he said.

Those who stay 'risking their lives'

Emergency Services Minister Kim Wells said there was a total recreational fire ban across the state, where there have been an estimated 12 arson attacks in the past 24 hours, and he urged people to heed evacuation orders.

"If the fire danger rating is severe, extreme or code red you are risking your life and that of your families if you choose to stay behind. The message is very clear -- leave and live," he said.

Wells said a second major fire in the Gippsland region could reach 500,000 hectares in size and take some time to bring under control.

In the Grampians, people started evacuating the holiday town of Halls Gap on Thursday evening after seeing a "big red glow on top of the mountain," said Rohan McDonald, owner of the Halls Gap Lakeside Tourist Park.

"We are covered in smoke, there is a massive plume that looks like an atomic bomb has gone off over the top of the mountain," he said.

In neighbouring South Australia state there were 67 fires going, four of which were declared emergencies, and a forecast lightning storm could complicate firefighting efforts, the Country Fire Service said.

"We will get more (fire) starts, that's pretty much guaranteed," chief officer Greg Nettleton said.

A scorching heatwave that has brought successive days of temperatures in excess of 40 degrees Celsius (104 F) continued to take its toll.

In Victoria, more than 400 heat exposure cases have been reported since Tuesday and 51 heart attacks -- a significant increase on normal figures.

"For an hour there this morning we were doing one cardiac arrest every six minutes and we expect that this will continue until this weather cools down," said Ambulance Victoria operations manager Paul Holman.

Melbourne was expected to register temperatures around 44 degrees Celsius on Friday while the South Australian capital Adelaide was set for 42C, its fifth straight day over 40 degrees, before dropping considerably on Saturday. The temperature reached 44C on Thursday in Adelaide.