Firefighters rushing to assist at a shocking fatal crash in Sydney’s south yesterday were horrified to discover the man killed was one of their own.

Drew Cullen, 39, died when the car he was travelling in collided with a truck and another vehicle on the Heathcote Road bridge over the Woronora river about 4pm.

Police are still investigating what caused the smash, but the off duty firefighter's car was completely crushed by the truck during the collision. The truck’s driver, a 42-year-old man from Yagoona, suffered a broken leg and the 28-year-old Wollongong woman in the third vehicle was taken to Bankstown Hospital with minor injuries.

Fire Commissioner Greg Mullins today said firefighters and emergency personnel who responded to yesterday’s crash were understandably shaken up after discovering their mate was involved.

Firefighters were shocked to find the deceased in yesterday's crash was one of their own. (9NEWS)

Mr Mullins said they would be offered counselling.

“The loss of a firefighter is deeply felt among the close-knit firefighting family,” he said in a statement issued today.

"My deepest sympathies and thoughts are with firefighter Cullen's family over their loss.

“When it involves one of our own, and in such tragic circumstances, it has a devastating impact.”

Mr Cullen worked at Fairfield Fire station. (9NEWS)

Flags at Fairfield fire station, where Mr Cullen worked, flew at half mast today in a show of respect.

The crash caused major delays for commuters last night, with Heathcote Road closed for much of the evening and traffic diverted through surrounding streets.

Heathcote Road was only re-opened this morning.