Authorities say at least 20 homes have been lost to fires raging south-west of Sydney on Thursday.

Earlier, NSW Premier Gladys Berejiklian put that number at 40 - half lost in Balmoral and Bargo and a further 20 in nearby Buxton.

However, the Rural Fire Service later confirmed 20 buildings had been lost to the blaze.

Some 2,500 firefighters are still battling more than 100 blazes across NSW, with half of them uncontained.

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AAP

It comes as three firefighters are treated for burns after their truck was overrun by one of the fires.

The graph shows the dramatic 17 degree drop in temperature from 42.4 at 2:03pm to 25.4 at 3:30pm as the southerly came through #Sydney Airport this afternoon. The southerly is now moving towards the Central Coast and Hunter with gusty winds and cooler temperatures. pic.twitter.com/eyDxEhDi77 — Bureau of Meteorology, New South Wales (@BOM_NSW) December 19, 2019

NSW Rural Fire Service Commissioner Shane Fitzsimmons said two male firefighters - aged 36 and 56 - had been airlifted to a burns hospital with serious injuries while a 28-year-old female was transported by road to hospital.

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AAP

A further two firies were treated by paramedics nearby for smoke inhalation after fighting the Green Wattle Creek blaze west of Bargo.

The commissioner said the crew was in one of many dozens of trucks in the area working to protect homes on Thursday when they were "overrun and enveloped by fire".

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"Those injuries are reported to three people in particular - two males with face and airway burns and some other burns to different parts of the body such as arms, elbow, upper chest and a leg," Mr Fitzsimmons told reporters in Sydney.

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"The third person, she is suffering some smoke inhalation and minor burns."

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The RFS commissioner said there were reports of "many structures and many buildings being impacted including some homes".

"There's everything from people's homes, people's businesses, there are poultry farming operations ... there are lots of buildings, including people's homes and livelihoods that have been impacted by this," Mr Fitzsimmons said.

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'In the vicinity of dozens" of buildings had been destroyed, he added.

"What that is in terms of homes, sheds, facilities - I just simply don't have that detail at this stage. It's too fast and unfolding as we speak."

Severe to Extreme Heatwave conditions across inland NSW today as hot air from central Australia is drawn into NSW by northerly to westerly winds.



Max temps will reach the mid-to-high 40s across most of NSW today with Penrith expecting to hit 45 °C https://t.co/tgZ0eqBSwm pic.twitter.com/nOMuV1XF1u — Bureau of Meteorology, New South Wales (@BOM_NSW) December 19, 2019

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The ABC has reported dingoes are running loose in the area, after escaping from the Bargo Dingo Sanctuary.

A number of dingos were evacuated from the site before the flames approached.

Resident Ray Hicks described the moment the firestorm hit.

Brad Wilson from a nearby wildlife sanctuary packed his car with dingoes and possums and just left @SBSNews #NSWfires #NSWbushfires pic.twitter.com/Bkkh4An4Gn — Charlotte Lam (@lottie_lam) December 19, 2019

"It was horrific, just a red glow all around, and black, the wind was that horrific it just blew straight over the top of us," he told the Seven Network.

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Another man told Seven he couldn't believe his home was saved: "I had my truck with water and I was pumping but I just had to leave. It was just a massive firestorm come though - it was like a wall of fire - I don't know how they (the RFS) saved the house."

The RFS said there were reports of 20 homes being lost but because the Green Wattle Creek fire was still active exact figures would not be known "for a number of days".

While the RFS said "fire activity has increased" at the Gospers Mountain blaze.

"Embers are being blown ahead of the main fire," it said.

Alarmingly conditions are set to worsen on Saturday after some possible respite due to a southerly change that swept up the NSW coast on Thursday evening lowering temperatures.

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Earlier in the day, Sydney's CBD reached a high of 39.3C while Sydney Airport and Penrith hit 42.5C. Elsewhere, White Cliffs topped 47.1C, Mildura recorded 45.2C and Broken Hill and Wilcannia reached 45.1C.

Air quality was hazardous across Sydney and much of the state on Thursday. It's forecast to be poor in the city on Friday.

Bureau of Meteorology acting NSW manager Jane Golding said Friday will be cooler too - with a top of 26C forecast for Sydney - but the whole state will heat up again on Saturday.

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Greenpeace remains concerned that bushfires are extremely close to the Springvale coal mine near Lithgow and the Tahmoor coal mine southwest of Sydney.

"Coal is extremely flammable, meaning any fires could likely burn for weeks, emitting toxic fumes which will aggravate the already dangerous levels of air pollution across NSW," Greenpeace Australia's research head Dr Nikola Casule said in a statement.

A statewide total fire ban remains in place until at least midnight on Saturday.

A statewide total fire ban will remain in place until at least midnight on Saturday and Premier Gladys Berejiklian on Thursday declared a week-long "state of emergency" granting special powers to the Rural Fire Service.

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There will be severe fire danger in the central ranges on Friday while elsewhere it will be very high or high.

Mr Fitzsimmons said Australian Defence Force aircraft, logistics and operations expertise will help fight the NSW fires over the coming days.

"We know this is a precious time of year for families to connect and unite and spend time with one another but I would really reinforce the message of the premier to re-evaluate, consider and assess your travel options," he said.