Properties have been lost to an emergency-level bushfire in Victoria, as crews scramble to contain the growing blaze ahead of catastrophic fire conditions.
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.
It comes as the most intense heatwave forecast to hit Australia in six years descends on southeastern Australia.
Victoria's Country Fire Authority chief officer Jason Heffernan says there are early reports of property losses in Longwood.
"We're getting teams out today to assess that, but early reports coming in to us here indicates that we have some degree of losses," he told ABC TV on Thursday.
CFA crews have confirmed damage to horse paddocks in the region.
A total fire ban is in place for the Mallee, North Central, North East and Northern Country regions on Thursday.
South Australians will continue to endure scorching temperatures that could peak in the high 40s in parts of the state on Thursday after sweltering in 40C heat the day before.
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.
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 $A60,000.
"To damage a sign that alerts the community to the fire danger rating is completely reckless and dangerous," Detective Acting Sergeant Leah Booth said.
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 would be 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 blazes before they could take hold, Forest Fire Management Victoria chief fire officer Chris Hardman 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.
