Extreme heat tipped for Hobart on Friday

Hobart residents are set to swelter through extreme heat on Friday with the temperature tipped to reach almost 40C in the city.

The Bureau of Meteorology (BoM) forecast a temperature of 38C in Hobart, before a cool change and possible thunderstorm move through Friday evening.

At 7am (AEDT), the temperature was already 17.4C in the city, which was four degrees above the average, BoM forecaster Anna Forrest told AAP.

"It's starting off well above average for this time of the morning, and I'd say it's probably going to stay there," Ms Forrest said.

Strahan, in the state's west, was the hottest location at 7am, recording 27.8C.

A total fire ban has been declared for the whole state.

Ms Forrest said the extreme heat was being caused by a "high pressure system sitting off the NSW coast".

"That's dragging air down from inland Queensland, NSW over Victoria and Tasmania, it's just hot northerlies over the whole of the south of Australia," she said.

But she expected temperatures to be "quite cool" on Saturday after the cold front hit late on Friday.

Ms Forrest forecast a maximum of 22C for Hobart on Saturday.

"That's closer to normal," she added.