Emergency warnings have been declared across two fire grounds on Thursday, near Mount Lawson state park on the Victoria/NSW border, which has burnt more than 4500 hectares, and at Longwood in northern Victoria that has burned in excess of 10,000ha.
Conditions throughout the day meant both fires had been active and volatile, State Control Centre spokesman David Nugent said.
"It's hot and windy. Conditions are dry and that's meant that our fires have been really active over the last few hours," he said.
Mr Nugent said heat and smoke from both fires had generated their own dangerous weather conditions and were producing their own lightning, meaning new fires could start.
Little Yarra CFA captain Peter Cookson fought the Longwood blaze overnight, describing the conditions firefighters faced as they defended homes and properties.
"We were on properties that were surrounded by fire ... trying to protect the houses from burning down because the fire was so intense everywhere," he told AAP on Thursday.
Mr Cookson said farmers had lost substantial amounts of grazing land.
Country Fire Authority chief officer Jason Heffernan said a severe and unstable air mass had formed above Victoria, warning the Longwood blaze would not be contained before Friday's dire conditions.
State Control Centre spokesperson David Nugent told AAP there were reports of building loss in the Longwood area but details were not available.
The Hume Highway has been partially closed as the Longwood fire continues to burn, with authorities warning the incredibly dynamic blaze could spread in multiple directions.
Friday's fire danger will be the worst across Victoria since Black Summer in 2019/20.
Authorities on Thursday described the upcoming weather conditions as "very, very dire", with forecasts of wind gusts up to 100km/h.
A total fire ban is in place for all of Victoria, with most regions given a "catastrophic" fire danger rating as temperatures are forecast to exceed 40C across much of the state.
Mr Heffernan urged people to leave before 7am on Friday.
Thousands of campers at popular holiday destinations have been ordered to evacuate, with public land across the state's southwest, Wimmera, north, central and northern country set to close.
Campers along the Goulburn, Campaspe and Murray rivers, Lake Eildon and surrounds, as well as the Grampians and Great Ocean Road must leave before 7am on Friday.
"It is not survivable if you are caught in a bushfire under those conditions," Forest Fire Management Victoria chief Chris Hardman said.
"You will be killed before the fire gets to you under these conditions."
At least 450 schools and kindergartens providing holiday programs will also close.
South Australians are also enduring scorching temperatures expected to peak in the high 40Cs across parts of the state, with most areas facing severe heatwave conditions.
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 39C on Friday.
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.
29°C