Two NSW Rural Fire Service (RFS) volunteer firefighters have died while battling bushfires south-west of Sydney.

Emergency services were called to the town of Buxton just before 11.30pm on Thursday, where an RFS truck travelling in a convoy is believed to have hit a tree and rolled off the road.

The NSW Rural Fire Service (NSW RFS) confirms that two NSW RFS volunteer firefighters have died tonight near Buxton, in south western Sydney.https://t.co/cDbysmaU1r #NSWRFS — NSW RFS (@NSWRFS) December 19, 2019

Police said the driver and the front passenger, both volunteer firefighters, died at the scene.

Three other firefighters received treatment for their injuries, and a crime scene has since been established.

An RFS spokesperson confirmed the deaths early on Friday morning.

"The Service’s thoughts are with all the firefighters family, friends and fellow brigade members," the spokesperson said.

"This is an absolutely devastating event in what has already been an incredibly difficult day and fire season."

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Addressing the media on Friday morning, NSW RFS Deputy Commissioner Rob Rogers said the firefighters were driving in dark and smokey conditions, making it difficult to spot fallen trees on the road.

"It just highlights (the) conditions that people are operating in, just how dangerous it is and particularly after the main fire front passes through, these trees just get burnt and they just can come down at any particular time," Mr Rogers said.

The firefighters, who have yet to be identified, were volunteers with the Horsley Park Rural Fire Brigade, based in western Sydney.

A day before the tragedy, the brigade posted dramatic photos on Instagram showing the intensity of the flames as its crews strengthened containment lines in the Blue Mountains.

Mr Rogers said the tight-knit brigade would be hit hard by the loss which would also be felt by the rest of the firefighters battling blazes.

But as dozens of fires continue to rage and threaten homes, he said firefighters had to "put that aside and keep going".

The incident came at the end of an exhausting day for firefighters, during which they fear up to 40 homes could have been lost in Buxton, Balmoral, Bargo and surrounding areas.

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

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Earlier on Thursday, three firefighters were treated for burns after their truck was overrun by one of the fires.

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 the hospital.

A further two firefighters 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".

"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.

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

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."

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.

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".

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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.

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.