Unpredictable and catastrophic fire conditions are putting thousands of residents in danger with campers warned the conditions are “not survivable”.

Three “leave immediately” emergency warnings remain in place for a significant blaze burning at Longwood, in northern Victoria.

There are fears the heat and smoke from the 2000-hectare fire will generate its own thunderstorm, with the potential for dry lightning.

“There is severe, unstable air mass above Victoria,” Country Fire Authority chief officer Jason Heffernan told reporters on Thursday.

Mr Heffernan confirmed the blaze continued to grow and would not be contained before the dire fire conditions.

State Control Centre spokesperson David Nugent told AAP there were reports of building loss in the Longwood area but details of the losses were not available.

Mr Nugent said fire had forced the partial closure of the Hume Highway.

Another bushfire continues to burn more than 3000 hectares at Mount Lawson on the border of Victoria and NSW.

“The conditions at the Mount Lawson fire are absolutely challenging,” he said.

“It’s hot, it’s steep and it’s absolutely difficult.”

Victoria last recorded catastrophic fire danger ratings across multiple districts on November 21, 2019, soon before the deadly Black Summer bushfires.

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.

Mr Heffernan said the fire had the potential to spread rapidly on Friday.

“Do not wait for a warning,” he said.

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.

The warning applies to those camping along the Goulburn, Campaspe and Murray rivers, Lake Eildon and surrounds, as well as the Grampians and Great Ocean Road, who 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 as authorities plead for people to leave.

South Australians will also endure scorching temperatures that could peak in the high 40Cs 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.

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.

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 rising, 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.