A hot air mass that has delivered scorching temperatures across large parts of Western Australia is expected to move across the country ahead of an approaching cold front, the Bureau of Meteorology says.

"The combination of these really warm conditions and also an increase in humidity is going to lead to very warm daytime temperatures and very warm night-time temperatures as well," BOM meteorologist Diana Eadie told AAP on Wednesday.

"As a result we're expecting severe to extreme heatwave conditions to develop across much of the southeast of Australia."

Temperatures are set to exceed 40C in Adelaide, Canberra and Melbourne, while some places such as Cummins in South Australia, Echuca in northern Victoria and Griffith in inland NSW are expected to reach 45C.

Ms Eadie said many areas will experience uncomfortable overnight temperatures including Adelaide where it will not drop below 28C on Friday.

"That's why we're seeing those severe to extreme heatwave conditions because when you don't get those temperatures dropping off overnight, it doesn't allow the body to recover."

Ms Eadie said the heatwave will bring elevated fire dangers, peaking on Thursday in South Australia, Friday in Victoria and Tasmania, and Saturday in fire-affected areas including the ACT and southern parts of NSW.

She noted humidity levels will be quite high.

"Whilst we'll see those warm temperatures and strengthening winds, it's not as dry as what we've seen with previous events which is why at this stage we're only forecasting severe fire dangers.

"It's not quite as dangerous as previous situations just because we do have that moisture over fire-affected areas."

Australian Associated Press