Towns and cities across multiple states are facing oppressive temperatures from Wednesday, with some places sweltering above 40 degrees for multiple days in a row.

More than half of Australia is set to melt in what could be the worst heatwave since the devastating Black Summer bushfires of 2019-2020 that killed more than 30 people.

The weather bureau is warning of extreme fire conditions across multiple states as temperatures top 45 degrees in parts of the country.

BOM heatwave warnings have been issued for NSW, Victoria, South Australia, Tasmania, WA and the ACT, variously affecting areas from Wednesday to Saturday.

South Australia and Victoria will be the first in line to experience the sweltering conditions on Wednesday.

Temperatures across all of SA are set to reach the low- to mid-40s, with Adelaide set for a maximum of 42 degrees and some inland and southeastern parts pushing 47 degrees.

Victorians will steam in temperatures reaching the low- to mid-40s, with Melbourne reaching 42 degrees and inland parts through to the north reaching 46 degrees, as severe heatwaves build and extend statewide.

“This is not just a normal burst of summer heat,” said BOM meteorologist Miriam Bradbury.

“These are significantly dangerous conditions which are likely to impact even those who are young and fit as well as those who may be more vulnerable to the heat.”

It will also be a warm day for Tasmania with low to severe heatwaves building in the state’s northeast.

The heat is expected to move into the ACT and NSW heading into the weekend, with the capital expected to peak at 42 degrees on Friday while Sydney reaches 39 degrees on the final heatwave day on Saturday.

Weatherzone said Adelaide and Melbourne would be in the thick of this week’s hot air mass, which has spread to the southeast of the country from WA.

Both cities are expected to top 41 degrees on Wednesday, with Adelaide tipped to hit 39 degrees on Thursday and Melbourne 38 degrees on Friday.

Sydney could have four days over 30 degrees from Wednesday, peaking at around 39 degrees in the city and 44 degrees in the western suburbs on Saturday, says Weatherzone.

Renmark, in SA, is bracing for 44 to 46 degrees from Wednesday to Friday, predicts Weatherzone.

Mildura, in Victoria, is forecast to reach 44 to 45 degrees on each of these three days.

The heat will combine with dry air and blustery winds to cause dangerous fire weather in parts of southern Australia, particularly SA, Victoria and southern NSW.

Total fire bans in place for the Wimmera, South West and Central regions, including Melbourne and Geelong for the first time this summer.

Victoria’s Emergency Management Commissioner Tim Wiebusch is warning of challenging extreme heatwave conditions “not seen since 2019/20”.

He said there is a very real risk of serious fires starting off the back of underlying dry conditions.

Country Fire Authority chief executive Jason Heffernan warned there aren’t many parts of the state immune to the fire conditions over the next 72 hours, after good rainfall led to prolific grass growth, adding to ground fuel.

“The fire seasons are starting earlier. They’re lasting longer, and we are seeing greater fire events occurring throughout that period,” he said.

Weatherzone said Friday would be a dangerous day for fires in Victoria, as temperatures in the low to mid-forties combine with wind speeds possibly exceeding 40kmh and gusts exceeding 70 kmh.

“A wind change will further exacerbate the risk of fires becoming difficult to control in Vic on Friday.”

The conditions are set to be the worst since the catastrophic 2019-20 summer bushfires that burned through millions of hectares.

“Our main concern is the three-day period running Wednesday through Friday,” senior forecaster Dean Narramore said.

“We’re likely to see extreme heatwave conditions over a huge part of NSW and northern Victoria, practically everywhere between Melbourne and Sydney and extending well inland.

“It is looking like … the most significant burst of heat for southeastern Australia, particularly in terms of multiple days in a row of mid-40 temperatures, since the summer of 2019-2020.”

Millions of residents in NSW and Victoria are bracing for extreme heatwave conditions from Wednesday to Friday.

“We’ll see temperatures really jump up to our populated parts of southeastern Australia, with even our coastal areas looking at temperatures in the low 40s,” Narramore said.

“That does include Adelaide and Melbourne and everywhere in between.”

Soaring temperatures are expected to ensure record spikes on the electricity grid as Australians crank up the air conditioning.

“There is an increased risk of unplanned outages during prolonged heatwave conditions, due to an increase in demand on the grid,” Matthew Sweeting from energy supplier Ausgrid said.

The bureau defines a heatwave as three consecutive days of above-average temperatures.

The heatwave comes after another record year, with 2025 Australia’s fourth-warmest on record and temperatures 1.3 degrees above the 1961 to 1990 average.

“This follows Australia’s second-warmest year in 2024,” Climatology Specialist Nadine D’Argent said.

“South Australia and Western Australia both recorded their third-warmest years.

“Between January and March, and between October and December, large parts of Australia experienced heatwave conditions, reaching extreme severity at times.”

Australia’s warmest year was 2019, when the national annual average temperature was 1.51 degrees above average.

-with AAP