'Absolutely devastating': Two volunteer firefighters killed battling bushfire

Two volunteer firefighters have been killed in the battle to contain a fierce blaze that ripped through villages southwest of Sydney.

The NSW Rural Fire Service volunteers were in a truck believed to have been travelling in convoy near the town of Buxton late on Thursday when it hit a tree and rolled off the road.

The driver and front passenger died at the scene, police said, while three other firefighters were injured.

The fatal accident occurred at the end of an exhausting day for firefighters in which it's feared some 40 homes could have been lost in Buxton, Balmoral, Bargo and surrounds, as the Green Wattle Creek blaze tore through the Wollondilly Shire on Thursday.

"The Service's thoughts are with all the firefighters' family, friends and fellow brigade members," the RFS said in a statement early on Friday.

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

Earlier on Thursday, three firefighters were treated for burns after their truck was enveloped by the bushfire.

Fire and Rescue NSW duty commander Inspector Kernin Lambert had described the conditions as deadly, even for firefighters.

"This was a horrendous fire," he told ABC TV.

"Around the Balmoral village, we had two fire fronts come together and they merged. We were experiencing firestorm-type conditions."

The Green Wattle Creek blaze remained at emergency warning level early on Friday, along with the huge Gospers Mountain blaze northwest of Sydney.

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The neighbouring Kerry Ridge blaze and a fire on the NSW South Coast at Currowan were downgraded to a "watch and act" alert.

Insp Lambert said the Green Wattle Creek fire developed rapidly before it bore down on the village of Balmoral.

Crews faced winds over 100km/h and 60-metre-high flame fronts. The fire was so fierce it sucked the oxygen out of the air.

The NSW RFS officially says 20 homes may have been lost but RFS deputy commissioner Rob Rogers acknowledged there are reports 40 buildings were destroyed.

It's unknown how many were houses, he told ABC TV.

"We're going to have crews in the morning start going through that (damage assessment) forensically. We'll know more on Friday."

Final tally of homes lost could surpass 40

Insp Lambert anticipates the final tally will be more than 40 homes lost.

"It's a really high number ... definitely," he said on Thursday night.

"The sheer destructiveness of this front ... it got to the point where houses were heavily involved in fire and we had to move on to where we could save people's lives."

Crews also continued fighting the 420,000-hectare Gospers Mountain blaze on Thursday night after a southerly wind change drove flames toward Bell in the upper Blue Mountains.

Firefighters are hoping to make the most of lower temperatures across NSW on Friday before extreme weather returns on Saturday.

Bureau of Meteorology acting NSW manager Jane Golding says the next 24 hours will be cooler but the whole state will heat up again on Saturday.

Northwest winds mean smoke will return to coastal communities while Sydney's west could hit 45C or higher. The majority of the state away from the coast on Saturday will experience temperatures in the mid-to-high 40s.

A week-long state of emergency has been declared in NSW and a statewide total fire ban remains in place.

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