Three capital cities are in for another day of sweltering temperatures as a heatwave continues to affect parts of the country.

Sydney, Adelaide and Canberra are all forecast to hit 40C on Thursday, following the hottest Australian day on record.

The average national temperature climbed to 40.9C on Tuesday eclipsing the record of 40.3C set in January 2013.

This record could be broken again with intense heat expected across most of Australia for Thursday and Friday.

'Exceedingly concerning' for NSW

Dangerous fire conditions have been forecast as large parts of NSW swelter through a heatwave and bushfires rage across the state.

The Rural Fire Service has forecast extreme fire danger for the Greater Sydney, Illawarra and Shoalhaven and Southern Ranges areas on Thursday.

Firefighters will confront an "enormous challenge" in coming days as the temperatures soar and volatile winds whip up fires.

Firefighters will confront an "enormous challenge" in coming days. Credit: DEAN LEWINS / AAPIMAGE

Premier Gladys Berejiklian warned the state would face significant bushfire risk on Thursday and Saturday with the potential for "exceedingly concerning" winds to blow embers well ahead of fire fronts.

"It's going to mean very unpredictable fire conditions," she said.

RFS Commissioner Shane Fitzsimmons said fire behaviour is forecast to be "erratic and significant" due to the volatile winds blowing across the state.

"The heat will start moving through NSW on Thursday and conditions will worsen on Saturday," Mr Fitzsimmons told reporters.

"We can expect the winds to be up and conditions to be at their peak very early in the morning and we'll have high sustained fire dangers for something like 15 hours."

A statewide total fire ban is in place from Wednesday morning through to midnight on Saturday. Credit: BIANCA DE MARCHI / AAPIMAGE

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A statewide total fire ban is in place from Wednesday morning through to midnight on Saturday.

A stifling hot air mass and blistering heatwave conditions scorched central Australia as temperatures soared beyond 45C on Tuesday with the hot air set to be dragged into NSW by winds over the next few days.

The Bureau of Meteorology expects most of inland NSW will see temperatures in the mid 40s.

As the mercury climbed on Wednesday, the huge Gospers Mountain fire northwest of Sydney was upgraded to "emergency" level after it crossed the Bells Line of Road through the Blue Mountains.

As the sun went down the fire was downgraded to a watch and act.

There were some 100 fires burning across NSW on Wednesday evening with more than half of those uncontained.

Temperatures are set to reach as high as the mid forty degrees in New South Wales over the next coming days, with a total fire ban in place across all of NSW until Sunday. Credit: BIANCA DE MARCHI / AAPIMAGE

Brief respite for Victoria

Thousands across Victoria are without power as the state swelters, with just one day of respite from the hot weather before the mercury soars again on Friday.

Melbourne reached a scorching 40.1C just after 4pm on Wednesday, while the state's northwest cracked the mid-40s as Walpeup hit 44.3C.

Power went out out for about 3000 PowerCor customers in Werribee and surrounding suburbs about 4pm on Wednesday.

People flock to St Kilda beach as a heat wave sweeps across Victoria. Wednesday, December 18, 2019. Credit: DAVID CROSLING / AAPIMAGE

Another 1400 Ausnet customers in Alexandra suffered a two-hour power outage from 5pm to 7pm on Wednesday.

Across Victoria, power company outage maps showed more than 3000 customers were still affected by 7pm.

The Bureau of Meteorology forecast a southerly wind change to bring temperatures down overnight, with milder conditions across southern areas forecast for Thursday.

Bushfires rage in Queensland

There is a high fire danger across Queensland which is expected to worsen into the weekend.

Central areas of Queensland have been issued a very high danger warning for Thursday, as crews battle about 70 fires across the state.

Heading into the weekend all coastal towns from central Queensland to the border will have a very high fire danger, with parts of central and southern Queensland expected to be under severe danger.

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A heatwave which has gripped the country has made conditions very tough for firefighters, who battled through Australia's hottest recorded day on Tuesday.

Homes were evacuated north of Brisbane on Wednesday, with more than 30 crews fighting a blaze at Peregian Springs, Lake Weyba and Weyba Downs, near Noosa.

Warnings are in place for fires at Gin Gin and Lowmead in areas between Bundaberg and Gladstone. Residents in Mount Maria were told to prepare to leave as a fire burning on multiple fronts could worsen.

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Fires near Gladstone reportedly claimed two buildings, and three people were taken to hospital in stable condition.

Residents near Carneys Creek and Cainbable, in the Scenic Rim region west of the Gold Coast, were told to prepare to leave just before 6pm on Wednesday.

Fire bans in the state's southern local government areas of Toowoomba, Southern Downs, Western Downs and Goondiwindi, have been extended into the first week of January.

Beachgoers try to get relief from the heat at Glenelg Beach in Adelaide, Wednesday, December 18, 2019. Credit: KELLY BARNES / AAPIMAGE

South Australia bakes

South Australia will cop the brunt of Thursday's heat, with the capital forecast to top out at 44C, and the western areas to be worse.

The southern-central town of Wudinna is forecast to hit 49C on Thursday, along with the Western Australian town of Forrest, in the state's south east.

Burera of Meteorology forecaster Simon Timcke said heat which has been building in the middle of the country is moving south, creating the extreme heat.

A record-breaking heatwave is making its way across southeastern Australia, with temperatures set to exceed 40C in many parts of the country. Credit: KELLY BARNES / AAPIMAGE

The heatwave will only continue to intensify, with southern and central Australia experiencing temperatures up to 16C above average by Friday.

Maximum temperature forecast, Thursday December 19