A heatwave sweeping across Australia has engulfed the country's densely-populated southeast, boosting temperature records and prompting fire bans.

A week after Australia's hottest town, Marble Bar in the remote northwest, recorded its warmest day ever, the heatwave on Friday pushed the temperature in the southeastern city of Melbourne to a near-record 42 degrees Celsius.

Areas to the north of the city were expected to be hotter and windy, prompting a fire ban across Victoria, the country's second most-populous state.

Nine years ago, Australia's deadliest bushfires killed 180 people near cities forecast to experience temperatures as high as 46C on Friday.

"The conditions are there that if a fire was to start, it could be quite difficult to contain," said Tom Delamotte, a Bureau of Meteorology forecaster.

In Hobart, capital of the more southerly island state of Tasmania, usually the country's coolest, the temperature rose as high as 40C, two degrees from a January record.

Pictures on social media showed a striking dark-orange sky over the city as a forest fire swept through the nearby wilderness. Campers were evacuated from the affected area, the Australian Broadcasting Corporation said, but no injuries were reported.

The Tasmanian Fire Service was not immediately available for comment.

The city council of Shepparton, north of Melbourne, sent lifeguards to ask holidaymakers to avoid the direct sun at the city pool as temperatures soared to a January record temperature of 45C.

"Everyone knows it's hot, but sometimes we forget the obvious things," said Mayor Kim O'Keeffe.

Forecasters expect temperatures to cool later, but warned the heat was likely to return soon as Melbourne prepares for the Australian Open tennis tournament, which is due to start on January 14.

Tennis Australia, the sport's governing body, says it has upgraded temperature testing at the Melbourne Park sports centre and introduced a 10-minute break for the men's singles.

It has also adopted a five-step "heat stress scale" that lets referees suspend play under extreme conditions.

Australia's weather agency warned in November that the country was facing a "hotter than normal" summer. The warning came as farmers battled severe drought and a heatwave fuelled forest fires in the state of Queensland.