An uncontrolled grassfire north of Melbourne is growing in size as it rages for a third day.

The blaze at Flowerdale, about 90 minutes north of Melbourne, increased to about 800 hectares in size overnight.

A total fire ban is expected across Victoria on Friday, as temperatures soar into the high 30s and 40s.

On Wednesday, a 71-year-old Flowerdale man was arrested by investigators probing the cause of the fire. He was released pending further inquiries.

Victoria’s Country Fire Authority chief Jason Heffernan said although the fire had been downgraded to ‘watch and act’, the situation could quickly change because hot winds of about 70km/h were tipped to sweep through the area.

“We’re in a heatwave situation here in Victoria and I expect fire conditions to continue to escalate today [and], in fact, peak tomorrow,” Mr Heffernan told Nine’s Today show.

The Wimmera and south-west parts of the state face extreme fire danger.

“The next couple of days are going to be very important for Victorian communities, particularly those that live near fires,” Mr Heffernan said.

Extreme risk in South Australia

Victoria’s fire danger is mirrored across the border in South Australia, where extreme bushfire conditions have been declared as it continues to swelter through its worst heatwave in more than three years.

The Country Fire Service said the bushfire risk was extreme from the west coast to the upper south-east, including the Adelaide Hills with total fire bans in place.

The Bureau of Meteorology has forecast a top temperature in Adelaide on Thursday of 41 degrees, on day three of the five-day scorcher.

The mercury is also tipped to hit 41 degrees on Friday before a cool change sweeps through early on Saturday, dropping temperatures to the mid-20s.

That will make it the longest string of days of 35 degrees or more since December 2019.

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The conditions have prompted authorities to activate heatwave emergency plans, with the State Emergency Service urging people in the hottest regions to stay indoors if possible.

In some regional centres, temperatures will surge into the mid-40s, including Nullarbor, which is forecast to reach a top of 45 degrees on Thursday.

Community centres in Adelaide have been opened for rough sleepers and homelessness services are conducting outreach programs around the city.

The Red Cross will also provide free healthcare checks by phone.

-with AAP