The power supply in Victoria and South Australia is being put to the test as five of the states and territories face an extreme heatwave, after temperatures in some areas hit 40 degrees Celsius.

The extreme weather is forcing a tight power supply balance, and people are being urged to prepare for possible outages.

The Australian Energy Market Operator has activated its "reserve trader functions" in Victoria, which will see it effectively pay some heavy energy users to curtail their power output on Friday afternoon.

The businesses involved had volunteered to be part of the demand management scheme.

The power supply situation is also tight in South Australia, where the Market Operator has issued a level two Lack of Reserve notice, meaning there is a small buffer of surplus generation available.

A level three notice means unexpected load shedding blackouts are likely.

However, Energy Minister Tom Koutsantonis is not expecting to use the state's new backup diesel power generators.

Total fire bans have been declared in parts of South Australia, Victoria and New South Wales, with a severe heatwave forecast by the Bureau of Meteorology (BOM).

How are you keeping cool? Send us your heat-busting pictures at yourpics@your.abc.net.au

Victoria

Some Melburnians cooled off at Port Melbourne beach as temperatures soared on Thursday. ( ABC News: Chris Sonesson )

Victoria sweltered on Friday, with Melbourne, Avalon Airport and Laverton reaching 42C.

A cool change predicted to move through the state had brought temperatures down by 12C in metropolitan areas by 5:00pm.

But the BOM said it had been slow moving, and Victoria's Emergency Management Commissioner Craig Lapsley said northern Victorians were unlikely to get respite in the coming days.

"The change does not extend to the northern part of Victoria, and northern Victoria will remain hot overnight, tomorrow, and will continue over the weekend to reach into the 40 degrees," he said.

A number of fires are also burning around the state, but no communities are being threatened.

Extreme temperatures will also push players at the Australian Open to the limit, with a court temperature reading of 69C recorded during the match between Novak Djokovic and Gael Monfils.

Children cool down at Half Moon Bay at Black Rock in Victoria. ( ABC News: Danielle Bonica )

South Australia

South Australia was also sizzling, with the Eyre Peninsula town of Wudinna recording the highest temperature — a top of 47.4C.

In other parts of the state, Port Augusta hit 46.5C, Whyalla reached 46.4C and Lameroo and Tarcoola both had tops of 46C.

In Adelaide, the mercury climbed to 42.2C just after 12:30pm, after reaching a top of 41C on Thursday and 38C on Wednesday.

Monash University's Roger Dargaville, who is an expert on energy systems, suggested the state would be getting pretty close to hitting peak demand on its power grid.

"They have 3,000 megawatts of total capacity and they're heading for 2.7 today, so again getting pretty close," he said.

He said a number of measures have been implemented to prevent the statewide blackout experienced in 2016.

"[Measures] such as a very large 100 megawatt battery, 250 megawatts of open-cycle gas turbines — so adding extra capacity into the grid to supply extra electricity when demand peaks," Dr Dargaville said.

"And otherwise providing additional emergency reserves, making sure there is extra power available when the demand goes high is a way the system can be secured."

The SA Government is not expecting to need its new backup generators.

There will be mild relief in Adelaide this weekend as temperatures begin to drop back to the mid-30s.

New South Wales

Sydney is set to be smacked with another heatwave just two weeks after Penrith recorded its highest temperature by a weather station in 79 years.

A severe fire danger rating has been issued for parts of NSW, including the Greater Sydney, Greater Hunter, Central Ranges, North Western and Northern Slopes regions.

About 20 fires are burning across the state, 10 of which are uncontained. There is no threat to lives or properties at the moment.

The Rural Fire Service's Greg Allan said firefighters are on high alert with temperatures in Western Sydney tipped to exceed 40C.

"We are likely to see high temperatures, hot dry conditions for most of today and right across the weekend," he said.

"So over the next few days it's really important that with our firefighters standing by and prepared, that residents are also doing what they can by knowing the fire danger rating and having a bushfire survival plan made and discussed ahead of time."

ACT

Authorities are warning Canberrans to keep cool and hydrated. ( ABC News: Greg Nelson )

Canberrans are also bracing for five days of hot weather, with temperatures expected to rise above 35C this weekend.

BOM has forecast temperatures in the ACT to hit 38C over the weekend and on Monday, while Sunday is tipped to be slightly cooler at a top of 37C.

Tasmania

A cool change has swept sweeping across Tasmania, bringing relief to some communities in the north east.

At 2:00pm the temperature hit 39C at St Helens, the highest recorded in the state on Friday.

An all-time record was set at Eddystone Point when the mercury reached 38.2C about 1:30pm.

Swimmers took to Launceston's Basin pool as temperatures hit 33 degrees. ( ABC News: Laura Beavis )



