A fire that engulfed a warehouse containing thousands of litres of flammable liquids continues to burn more than a day after it started in Sydney's south west.

NSW Fire & Rescue Superintendent Greg Rankin said crews had been at the industrial zone in Revesby overnight and were continuing to extinguish pockets of fire.

More than 100 firefighters fought from all sides to contain the massive blaze fuelled by chemicals stored in the warehouse on Friday.

There were a series of explosions that sent balls of fire and thick plumes of black smoke into the air.

Nearby factories were evacuated and two people were treated at the scene for smoke inhalation.

Paramedics remained on standby and HAZMAT crews monitored the air for chemical toxins.

While it was initially thought the fire started in a car parts manufacturing plant, Superintendent Rankin said they had now discovered it had broken out in a solvents and lacquer distribution warehouse on Fitzpatrick Street.

The warehouse has now collapsed in on itself but fires are continuing to burn inside it and there are concerns of further minor explosions.

"They have got a heavy excavator in there at the moment which is removing all the debris so they can get to these hot spots," Superintendent Rankin said.

"They have said it could take the rest of the weekend to finally get it all completed."

A second warehouse, containing industrial machinery, was also damaged by the blaze.

On Friday, NSW Fire & Rescue Commissioner Greg Mullins said the fire was not being treated as suspicious given the time of day and where it occurred.