Authorities are scrambling to contain emergency-level bushfires ahead of catastrophic fire conditions forecast for parts of Australia.
The most intense heatwave forecast to hit Australia in six years has descended on southeastern Australia.
South Australians will continue to endure scorching temperatures that could peak in the high-40Cs in parts of the state on Thursday after sweltering in 40C heat.
Most of the state faces severe heatwave conditions, with a total fire ban in the mid-north region.
After the mercury reached the mid-40s in inland and northern parts of Victoria and 40C in Melbourne, southern parts of the state will have a slight reprieve on Thursday, but northern areas will have no such luck.
Two emergency warnings remain in place for a blaze burning at Longwood in northern Victoria, with residents told to leave immediately before conditions become too dangerous.
Another bushfire continues to burn more than 1000 hectares at Mount Lawson on the border of Victoria and NSW.
A total fire ban is in place for the Mallee, North Central, North East and Northern Country regions on Thursday.
The ban includes the town of Koonoomoo, where police are investigating after a CFA fire danger sign was cut down on Wednesday.
Thieves stole the sign's solar battery before leaving behind damage estimated at $60,000.
"To damage a sign that alerts the community to the fire danger rating is completely reckless and dangerous," Detective Acting Sergeant Leah Booth added.
Victoria faces fire dangers in the upper end of extreme on Friday, with the districts of Wimmera, Northern Country and North Central forecast to meet catastrophic levels, State Control Centre spokesman David Nugent said.
"Catastrophic fire danger rating is the most dangerous set of conditions for us for firefighting," he said.
"If a fire starts and takes hold, lives and homes are likely to be lost."
Bureau of Meteorology forecaster Bri Macpherson said Friday was a significant fire danger day because vegetation dried out by lingering hot air combined with winds and dry thunderstorms increased the risk of ignition.
Ground crews were preparing for aggressive first attacks to douse fires before they could take hold, Forest Fire Management Victoria chief fire officer Chris Hardman said.
"If we can't catch those fires when they're small - and I mean really small, one tree burning through lightning - then those fires are going to get big and have dreadful impacts on the community," he said.
The heat is set to move into the ACT and NSW from Thursday through to the weekend, with the national capital expected to peak at 38C on Thursday and Friday.
Temperatures in parts of NSW were also on the rise, meteorologist Miriam Bradbury said, reaching the high 30Cs to low 40Cs as severe and extreme heatwaves took hold in the state's southeast, eastern and inland districts.
But even before Sydneysiders feel the brunt of the 42C peak on Saturday, people with asthma or other respiratory conditions are warned to avoid outdoor exercise because the heatwave is expected to cause poor air quality in the city's southwest and northwest.
This factor, which can exacerbate lung and health issues, generally builds during hot conditions, peaking in the mid-to-late afternoon and easing off as temperatures fall, according to NSW's Environment Department.
Last year, was Australia's fourth-warmest year on record.