Firefighters are nervously preparing as a heatwave, windy conditions and lightning create a perfect storm for some of the worst bushfire conditions in years.

A major heatwave across Australia's southeast is expected to continue pushing up the mercury with temperatures forecast to reach 41C in Melbourne and up to 47C in parts of inland Victoria on Friday. Western NSW temperatures are forecast to reach the late 30s and early 40s.

Emergency warnings were issued for bushfires in northeast Victoria and Longwood overnight while fire warnings were elevated to catastrophic in the state's Wimmera, northern country and southwest regions, while a total fire ban is in place across the entire state for Friday.

Little Yarra Country Fire Authority captain Peter Cookson, who has been fighting a blaze at Longwood in the state's north, said the conditions still made him nervous, despite his years of experience.

"It's all going to be a very bad day," he told AAP.

"We all prepare for these days, and if nothing happens, that is a win.

"But you can't say nothing will happen – with these things, they can all start from a spark."

Fire crews are continuing to attack two major out-of-control blazes, with the Longwood fire razing more than 28,000 hectares while flames near Walwa and Mount Lawson along the Victoria/NSW border have burned more than 9000ha.

Wind gusts of up to 90km/h and a severe thunderstorm risk could cause erratic fire behaviour, the Bureau of Meteorology warned.

A number of new fires have already sparked across the state due to dry lightning, State Control Centre spokesman David Nugent said.

"Under those catastrophic conditions, there is real potential for lives and homes to be lost," he told AAP.

The strong winds could also ground the firefighting air fleet.

"It's every chance at some point the aircraft may not be able to fly," CFA chief Jason Heffernan said.

"We can expect if a fire takes hold, and we see the fire spread the way they're going, they're going to be uncontrollable. They're going to be unstoppable."

Paramedics and first responders in catastrophic fire danger areas will be withdrawn on Friday and will only be redeployed to these zones for serious or life-threatening illnesses, Ambulance Victoria said.

The heatwave is expected to affect the ACT and NSW, where a total fire ban has been declared in the eastern Riverina, southern Riverina, southern slopes and Monaro alpine regions for Friday.

Temperatures will not be quite as high in NSW, with Sydney set to reach a maximum of 33C and Canberra expected to hit a high of 39C.

But things are set to worsen on Saturday with the harbour city expected to reach 42C, with even warmer temperatures forecast for the western suburbs.

NSW Ambulance chief superintendent Steve Vaughan urged residents to begin preparing for the hot conditions and stay cool, hydrated, and indoors during the hottest part of the day.

"Heatwaves put real pressure on your body, with consecutive days of relentless heat causing stress that builds up over time," he said.

As at early Friday, the Bureau was forecasting a high of 41 for Bourke on Friday and similar highs until at least next Thursday. Dubbo's top for Friday is expected to be 39, followed by highs of 40 on Saturday then 36 with a possible shower developing on Sunday. Orange will be relatively milder with a high of 33 Friday and again Saturday, then cooling slightly after that.