Firefighters will confront an "enormous challenge" in coming days as temperatures soar and volatile winds whip up dangerous bushfires burning across NSW.

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

"Not only are we going to experience very high temperatures on Thursday and Saturday but there's the exceedingly concerning wind conditions," she told reporters on Wednesday.

"We're going to have a number of wind fronts escalating the fuel, the fires burning, and the potential to have spot fires and embers travelling very long distances.

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

An exceptionally hot air mass will spread over Australia this week, possibly challenging records for individual locations, states and the country as a whole: https://t.co/znx3meDvkR pic.twitter.com/dB5QOd6Lyo — Weatherzone (@weatherzone) December 16, 2019

RFS commissioner Shane Fitzsimmons says 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."

At 8.30am there are 99 fires burning across NSW with more than half yet to be contained. Over 2000 firefighters will be in the field today.



Conditions are predicted to deteriorate tomorrow. What preparations are you making? What is your plan?#NSWRFS #NSWFires pic.twitter.com/PzRLtFOcBE — NSW RFS (@NSWRFS) December 17, 2019

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

Firefighters will work around the clock to contain fires before the worst of the conditions hit.

There were some 100 fires burning across NSW on Wednesday at midday with more than half of those uncontained, the RFS said.

Daytime temperatures in parts of NSW and the ACT are expected to exceed 40C through to Saturday while in the western half of the state the mercury could top 45C.

Wind gusts could top more than 100 kilometres per hour.

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Emergency Services Minister David Elliott says complacency is the biggest threat.

"Just because you haven't seen a fire at the end of your street, just because you can't see smoke in your suburb, doesn't mean that you are not at risk," he told reporters.

There would be 3000 firefighters working on Saturday to try and contain dangerous fires, the minister said.

Some 768 homes have been destroyed by bushfires this fire season, according to damage assessment figures released by the RFS on Wednesday.

Almost 300 homes have been damaged while just over 1700 outbuildings have been destroyed and another 730 outbuildings damaged.

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Climatologist Dr Blair Trewin said Australia may this week hit a record for its warmest day, exceeding the 40.3C average maximum temperature set in January 2013.

Victoria could record its hottest December day on Friday if centres in the northwest such as Mildura and Swan Hill reach a forecast 47C.

"We're expecting a lot of December records to be challenged, particularly in northwest, north-central Victoria some parts of northwest Victoria may get very close to records for any time of the year," Dr Trewin said.

Mildura is forecast to swelter through three days in a row of 45C and above

"That's only happened once before in 1939 so it is certainly a very significant heatwave, particularly for December," he added.

In western NSW, temperatures are expected to climb above 45C while Canberra is bracing for 43C on Saturday.

"For the ACT we're looking at five days in a row above about 37 degrees. Three of those days will be in the low 40s, so severe to extreme heatwave conditions," BoM acting NSW manager Jane Golding said.

Alice Springs is expected to reach 45C on Thursday, close to its hottest temperature of 45.6C, a record met twice last summer.

A statewide Total Fire Ban is now in effect through until midnight Saturday 21st December 2019, due to hot and dry conditions across NSW. If you see an unattended fire please call Triple Zero immediately (000). #NSWRFS pic.twitter.com/Ar9w17SgTT — NSW RFS (@NSWRFS) December 17, 2019

Parts of Queensland are still sweltering despite a cool change bringing relief to Brisbane after it hit 41.2C on Monday, its highest December temperature since 1981.

But the western town of Birdsville is forecast to remain in the mid-40s all week, peaking at 47C on Wednesday.

As the mercury climbs, authorities are warning people to prepare for the hot weather.

"Extreme heat kills more Australians than any natural disaster and can affect anybody, but those most at risk are older people, young children and those with medical conditions," Victoria's State Response Controller Gavin Freeman said.

He advised people to stay hydrated, keep cool, never leave kids or pets in hot cars and check in on others who may be at risk.