‘Tragedy’: Young dads killed in RFS crash

Tributes are beginning to fly for the two RFS volunteers killed last night when a fallen tree caused their tanker to roll as they fought fires south of Sydney.

Horsley Park Brigade Deputy Captain Geoffrey Keaton, 32, and firefighter Andrew O'Dwyer, 36, were both killed just before midnight in the crash.

Premier Gladys Berejiklian also paid tribute to the two men, calling them "heroes".

Three others are in hospital from the accident and two more firefighters are in a coma after being overcome by flames in a separate incident.

Firefighters are in for a tough day right around the country with Queensland facing terrible conditions and South Australia set to have its worst fire risk day of the summer with catastrophic conditions declared across the state.