Security footage has shown two explosions ripping through a factory about a minute after a large blaze broke out in Sydney's south-west.

CCTV vision from a car wrecking yard opposite the furniture factory in Greenacre showed the fire taking hold in the brick building about 3:10am today.

The furniture factory and an adjacent blinds and awnings factory have been destroyed.

Two explosions ripped through the factory about a minute later which blew out the building's glass door and windows and destroyed a metal roller door.

A furniture factory and a blinds and awnings factory were destroyed. ( ABC News: Johanna Nicholson )

The footage shows the owner leaving the furniture factory about 6:00pm the evening before.

Officers and detectives from Banktown police are investigating the causes of the fire, but said it was too early to say whether the fire was suspicious.

They said they would prepare a report for the coroner as a matter of routine, regardless of whether the fire is deemed to be suspicious.

Wall collapse at a fire in Greenacre. ( ABC News )

Firefighters said spot fires were still burning beneath the collapsed roof of the two factories and that it could be at least a day before the blaze is fully extinguished.

Fire and Rescue NSW said they would use a heavy excavator to tear down the damaged brick wall at the front of the furniture factory and remove the rubble, so they can put out the spot fires.

A waterproofing business in an adjacent warehouse received structural damage but firefighters were able to stop the blaze spreading to other buildings.

A number of buildings in the area were damaged. ( ABC News: Jessica Kidd )

About 20 fire trucks and 100 firefighters were called to the scene on Rosedale Avenue from surrounding suburbs.

Fire and Rescue New South Wales Superintendent Andrew Ticehurst said a partial wall collapse at the wrecking yard contributed to the fire spreading.

There were no injuries but some people were evacuated from surrounding businesses, he said.

"The fire is still burning in a couple of pockets in the factories where the fire was burning intensely earlier this morning," he said.

"Firefighters are now using some large ladders just to direct some water streams out over the rubble to try and extinguish the pockets of fire."

He said the fire had been contained and mostly extinguished and resources were being scaled back.