Two men critical from Sydney fire

Two workers from a car wrecking yard are fighting for life in hospital after being severely burnt in a fire at an industrial site in Sydney's southwest.

More than 100 firefighters fought the massive blaze, following initial reports of explosions, at the Revesby site on Wednesday afternoon.

Two employees, aged 32 and 40, were taken to the Royal North Shore Hospital where they remain in a critical condition, a NSW Health spokesman said on Thursday.

Fire crews worked overnight and remain on site to extinguish small pockets of fire in the car wrecking yard, Superintendent Greg Rankin of Fire and Rescue NSW told AAP.

The blaze is not being treated as suspicious.

"It's some type of accident but we haven't been able to exactly work that out," Supt Rankin said.

"We've got police and fire investigators coming down here and also some structural engineers to check the integrity of a couple of buildings."

An adjoining auto works factory and a gymnasium, which were affected by fire, are closed off.