Geoffrey Keaton, 32, and Andrew O'Dwyer, 36, from the Horsley Park brigade, were in a truck convoy near the town of Buxton late on Thursday when a tree fell into their path, prompting the vehicle to roll off the road.

The two men - both fathers of young children - died at the scene, while three other firefighters were injured and have been hospitalised. They had been able to independently free themselves from the crashed truck.

The fatal accident occurred at the end of an exhausting day during which it's feared some 40 homes were lost in Buxton, Balmoral, Bargo and surrounding areas as the Green Wattle Creek blaze tore through the Wollondilly Shire.

"I understand they were a pretty tight-knit group," RFS deputy commissioner Rob Rogers said on Friday morning.

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"This is quite a tight brigade and that will impact significantly on that group.

"At the end of the day, everybody feels for these people and their families ... people have got to try and put that aside and keep going.

"The immediate people around that crew, we will make arrangements for them to be excused from the fire ground if that's what they wish, but often firefighters like to just get back to doing what they do ... it's a coping mechanism as well."

Mr Rogers said commissioner Shane Fitzsimmons had spent the evening with the families of the dead firefighters.

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

AAP