Two volunteer firefighters were killed and three others injured in "the worst imaginable set of circumstances" when their truck rolled after being hit by a tree in Sydney's south-west.

Key points: The men who died, Geoff Keaton, 32, and Andrew O'Dwyer, 36, were both fathers to young children

The men who died, Geoff Keaton, 32, and Andrew O'Dwyer, 36, were both fathers to young children Their truck was part of a convoy near the Green Wattle Creek fire at Buxton

Their truck was part of a convoy near the Green Wattle Creek fire at Buxton Conditions have eased today, giving crews a chance to assess the number of properties lost on Thursday

Geoffrey Keaton, 32, and Andrew O'Dwyer, 36, both fathers to young children, were travelling in a New South Wales Rural Fire Service (RFS) convoy to battle blazes about 11:30pm yesterday.

RFS Deputy Commissioner Rob Rogers said the truck rolled on Wilson Drive, Buxton, killing the pair and injuring three passengers, who were taken to Liverpool Hospital.

RFS Commissioner Shane Fitzsimmons spent the evening with the families of those killed, while NSW Premier Gladys Berejiklian said the state had lost "two heroes".

The tanker rolled after it was hit by a falling tree near Buxton. ( ABC News )

Commissioner Fitzsimmons said the three injured firefighters were stable but in deep shock after what they witnessed last night.

The incident was on the front line of the Green Wattle Creek fire, which is still burning at emergency level.

A crime scene was established, and an investigation launched into the circumstances surrounding the crash.

Falling trees have been a constant problem across the fireground this season and the RFS said a number of tankers had been damaged.

A firefighter who was a passenger in the truck rollover arrives at Liverpool Hospital. ( ABC News )

The firefighters were from the Horsley Park brigade in Western Sydney, which Deputy Commissioner Rogers said was a very tight-knit group.

"This is going to impact significantly on that brigade," he said.

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

Prime Minister Scott Morrison Scott issued a statement this morning saying he would be returning to Sydney as soon as possible given the tragic event.

The incident came at the end of a dangerous day of fires and heatwave conditions across Greater Sydney, with at least 20 properties destroyed.

Dedicated firefighting families

Mr Keaton's family has long been involved in the RFS and his father was also volunteering yesterday to fight the Gospers Mountain fire.

Geoffrey's son Harvey and Mr O'Dwyer's daughter Charlotte are both 19 months old and were born just days apart.

Flags are flying at half mast at the Horsley Park Rural Fire Brigade and around the state. ( ABC News: James Carmody )

Commissioner Fitzsimmons said their families were in "extraordinary shock" and the Horsley Park brigade was at a loss.

"This is the worst imaginable set of circumstances … both of these men were very well-respected … they're a very community-focused brigade, they work together, socialise together," he said.

Geoff Keaton's helmet placed at a memorial outside the Horsley Park station. ( ABC News: James Carmody )

The commissioner said the entire RFS was "hurt to the core", but the firefighters were trying to stay focused as conditions are expected to be extremely dangerous over the weekend.

Deputy Commissioner Rogers said a lot of containment work needed to be done today to protect suburban areas.

"While this is an awful thing, it won't deter us from doing a lot of work today, trying to get a level of containment on these fires because [tomorrow] is going to be another awful day," he said.

20 structures lost yesterday

Temperatures have dropped significantly since yesterday's sweltering heat, with the Sydney CBD expected to reach a top of 26 degrees Celsius and 33C in Penrith.

But the reprieve will not last long, according to Bureau of Meteorology spokesperson Neil Bennett.

"Saturday's going to be exceptionally hot, with a forecast max of 47C [in Penrith]," he said.

Mr Bennett said temperatures were expected to fall as a southerly change came through from Saturday afternoon but it will not be much help for firefighters battling bushfires.

"We're certainly not out of the fire-weather scenario, even going into the middle part of next week," he said.

"Until we get some substantial rain into the fire areas, they're just going to smoulder away and then there's the risk of some flare-ups."

Authorities hope calmer conditions will allow crews to assess Thursday's damage. ( ABC News: Brendan Esposito )

Yesterday the RFS said at least 20 buildings in and around Bargo, near Picton, were destroyed by the Green Wattle Creek fire.

However, RFS deputy commissioner Rob Rogers said this was a conservative estimate, and that there were reports dozens of properties were razed.

"We're going to get people in there and they will just catalogue … and make sure we properly account for everything that's been destroyed and damaged," he said.

"We don't know if they're all homes or whether they're sheds, or a mixture of both."