Authorities are warning of very dangerous fire conditions as the heatwave sweeping much of the country threatens power supplies in New South Wales, and South Australia's power crisis rollson.

NSW, the Australian Capital Territory, Victoria and South Australia all experienced extremely hot weather yesterday, with the mercury climbing beyond 40C in many locations.

"Temperatures are sitting around 42 or 43C currently near Victoria's northern border and tracking pretty close to the forecast," senior forecaster Rod Dixon said yesterday afternoon.

The heat is forecast to continue through the weekend before things cool off early next week.


In NSW, the Australian Energy Market Operator (Aemo) warned of a possible power shortfall amid record demand as people crank up their air-conditioners.

The state's Energy Minister, Don Harwin, urged people to do what they could to save power as Aemo talked to power generators to try to ward off load shedding.

"The NSW Government is doing everything we can to ensure a power outage is avoided but we encourage the community to reduce their energy use where possible," Harwin said.

Load shedding caused by peak power demand resulted in up to 90,000 homes and businesses being deliberately blacked out in South Australia on Wednesday night.

Aemo ordered a second generator at Port Adelaide to be fired up on Thursday, preventing a second night of outages.

But the stress on the system will persist, with Adelaide and many parts of SA around 40C again yesterday.

The heat was also having an effect on health, with 39 people admitted to SA hospitals with heat-related illnesses in the past two days.

Amid growing frustration with the national electricity market, SA Premier Jay Weatherill says his Government will take dramatic action over the state's energy future.


"We are going to make our own decisions, we are going to take control of our own future and our own destiny," the Premier said on Thursday, but he did not detail how he would achieve that.

The fire danger will be severe or extreme across large parts of the country in coming days.

NSW Rural Fire Service commissioner Shane Fitzsimmons said the danger would increase across southern parts of the state.

He said winds would pick up today and a catastrophic fire danger could develop in some areas by tomorrow.

The Hunter region will cop the brunt of the extreme heat through the weekend, with Singleton and Cessnock topping the predicted temperatures with 46C today and 43C tomorrow.

Hardest hit will be towns in the state's far north and west including Bourke and Ivanhoe - tipped to reach 47C and 48C during the weekend.