Warnings have been issued for parts of Victoria and South Australia as firefighters battle blazes in both states.
The South Australian Country Fire Service is battling several grass fires across the state, including in the Mount Lofty Ranges, as the state grapples with catastrophic fire conditions in two districts.
In Moyston, in western Victoria, firefighters have stopped the head of a fast-moving grass fire which has burned more than 2000 hectares, the ABC reported.
Earlier today, authorities warned Victorians and South Australians about risky fire conditions today and tomorrow, with temperatures soaring into the forties.
There were catastrophic fire danger warnings in two South Australian districts, the Mount Lofty Ranges and Lower South East.
In Victoria, there was an emergency warning for communities south of Moyston - later downgraded to a watch-and-act message. Residents were advised to stay close to shelter in case the conditions changed suddenly.
Communities brace for high temperatures
Temperatures were forecast to climb to 43°C at Mildura, Swan Hill, Horsham and Echuca in Victoria today and tomorrow, while 38-40°C was expected in Melbourne.
"We need to make sure that people take the next two days seriously across Victoria," Emergency Services Commissioner Craig Lapsley said.
"It's going to be hot. It's going to be dry. It'll be windy ... The conditions are conducive for fires to run and if fires start they could be uncontrolled very quickly.
"We'll see severe and extreme fire danger readings in most districts particularly in the west and central parts of Victoria."
Total fire bans were declared for eight of nine districts in Victoria on Friday, excluding East Gippsland and, on Saturday, the entire state will be under a total fire ban.
That means it is illegal to start a fire in the open, people must not use equipment that could start a fire and campers must not light solid fuel fires.
Scott Williams, a senior forecaster at the weather bureau, said it was a bit early in the season.
"But nonetheless it's still going to be a very dangerous for fires on Saturday and in the east of the state too, because the winds will pick up down there," he said.
South Australia was also set to swelter, with temperatures set to hit 42°C in Adelaide today.
Elderly and infirm at risk
Ambulance Victoria's Paul Hulman said the biggest danger in terms of heat stress was that overnight temperatures would remain high, meaning people would not have time to recover.
That put the elderly and infirm at particular risk.
"Those members of our community, the elderly and particularly those with pre-existing illnesses, will have trouble overnight. The body doesn't have enough time to recover."
He said those people should be checked on in the morning.
"Last weekend, we had nine children locked in cars. Now that was a cool weekend," he added
"If we have nine children locked in cars this weekend those children will be in trouble."