Sydney will swelter through its hottest December night in 44 years, with the mercury not expected to fall below 25C.

The city also faces the prospect of its hottest back-to-back days in 10 years.

Temperatures are forecast to hit 36C in the CBD and 39C in the west on Tuesday and climb to 39C and 38C, respectively, on Wednesday.

If those temperatures are realised, Sydney will record its hottest consecutive days since October 2006, when temperatures reached 36.7C and 36.2C, according to Bureau of Meteorology.

“We are going to have generally hot and dry north westerly winds associated with these warm temperatures,” BoM forecaster Sarah Chadwick said.

The winds will also increase the risk of bushfire fires so the NSW Rural Fire Service has declare total fire bans across much of the state.

A severe fire warning is also in place for the lower central west plains and the entire Riverina district.

Heatwave hits other cities

The sharp blast of heat also has Canberra, Melbourne and Adelaide sweating, with the mercury soaring above 30C before lunchtime.

Temperatures were forecast to peak at 36C in Adelaide, with Melbourne and Canberra slightly lower at 34C and 33C respectively.

John Turnbull, from the Bureau of Meteorology’s Extreme Weather Desk, said hot, dry winds drawn down from northern Australia were causing the temperature spikes.

Crowds have been flocking to beaches since the early hours in Sydney. Photo: AAP.

“The change will move through South Australia today and Victoria tonight with milder conditions for both states tomorrow,” he told AAP.

“Sydney and Canberra will be warm to hot today and tomorrow and cooler by Thursday.”

Fire bans in three states

The hot weather has led to total fire bans being declared across 10 districts in South Australia, seven in NSW and three in Victoria.

Air conditioner use is likely to increase, especially across greater Sydney, and energy providers are reassuring customers they are prepared for the increased demand.

“On days like today, electricity consumption can increase by 50 per cent when temperatures approach 40C with the use of air conditioners,” Endeavour Energy spokesman Peter Payne said.

Meanwhile, hot weather can be dangerous for vulnerable children and adults, and the Red Cross wants everyone to keep an eye on their neighbours.

“Remember to check on your neighbours because older people, pregnant women, children, those with a disability and people taking medications are among those who are more at risk,” a spokeswoman said.

A cool change is due on Wednesday evening from about 4pm or 5pm and will continue on Thursday when temperatures are forecast to drop to around 19C, the bureau said.