Work warning as states swelter and bushfire risk soars

Heatwave warnings are current for parts of all mainland states, prompting extreme to catastrophic danger alerts to be issued for parts of South Australia, northern Victoria and outback NSW.

Mildura, Hay, Ivanhoe and Bourke were among the towns expecting 45C, while South Australia's Renmark was due to hit a peak of 46C.

Temperatures in outer suburban Sydney and Melbourne were expected to top 35C, while Canberra was poised for a high of 34C.

The South Australian Country Fire Service warned Friday would be "one of the most significant fire weather days in several years".

Adelaide hit 29.6C before 7am on Friday, as temperatures across the state head towards the mid-40s.

Thousands of customers were without power in the early afternoon, the majority of them in areas east of Port Augusta, as the extreme weather conditions wreaked havoc on electricity networks.

Strong winds and dry lightning are adding to the state's fire risk.

Catastrophic fire danger ratings were declared for the mid-north, Flinders, Yorke Peninsula, Riverland and eastern Eyre Peninsula districts, while five regions face extreme fire danger.

Any fires that did start would be "incredibly difficult" for firefighters to control, CFS chief officer Brett Loughlin said.

"The dry lightning creates a real risk of fire starting anywhere these thunderstorms occur, unfortunately the forecast is a broad area of dry lightning and thunderstorm activity," he said on Thursday.

The service secured a large air tanker from the Commonwealth to help with firefighting operations, but Mr Loughlin said conditions could hamper its use.

"I want to stress, the forecast winds tomorrow are such that there is likely to be times when an aircraft will be unable to safely fly and fight fires," he said.

"It is crucial that people do not rely on firefighters and aviation assets to save them."

After Friday's heat, temperatures will dip to the low to mid-20s across much of SA on Saturday, with Adelaide forecast to have up to 60mm of rain over the weekend.

A total fire ban is in place for several NSW areas west of the Divide for Friday, ahead of even warmer conditions on Saturday, when Sydney's west could hit 44C.

Almost 20 NSW government schools were closed on Friday due to bushfire risk.

Meanwhile, the mercury didn't dip below 30C overnight in the outback town of Wilcannia as it sweats through a forecast 10-day streak of 40C days.

High temperatures sparked a warning to employers to protect their workers and develop a workplace heat-management plan to ensure safety.

"We don't want to see anyone needing medical attention due to heat exposure," SafeWork NSW head Trent Curtin said on Friday.

"Workers are reminded to listen to their body when the temperature rises and to drink plenty of water.

"It is vital you do not replace water with soft drinks, energy drinks or coffee as these can dehydrate you further. "

Parts of Queensland, West Australia, the Northern Territory and ACT have also copped soaring temperatures that are expected to intensify over the weekend.

Australian Associated Press