The accident occurred when the truck crashed into a tree then rolled off the road during an operation late Thursday at Buxton, about 100km southwest of Sydney, the NSW Rural Fire Service said.

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

Prime Minister Scott Morrison paid tribute to the firefighters, saying their sacrifice would always be remembered.

“They were bravely defending their communities with an unmatched spirit and a dedication that will forever set them apart amongst our most courageous Australians,” he said in a statement.

NSW has declared a state of emergency as an unprecedented heatwave fans bushfires that have destroyed homes and smothered huge areas with a toxic smoke, including Australia’s largest city Sydney.

Thousands of firefighters are trying to contain the blazes amid record temperatures nearing 50°C in places.

Morrison warned conditions were set to worsen.

“These fires and heat conditions are horrendous, and there are still difficult days ahead, with Saturday predicted to be the most severe day, with extreme temperatures and wind making conditions very difficult for fire crews,” he said.