Firefighters are continuing to work to extinguish a large factory fire at Greenacre, in Sydney's south-west.

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

Some firefighters are still at the scene but the blaze is mostly extinguished.

Earlier, it was believed the fire broke out in a car wrecking yard but witnesses said it appeared to start in a furniture factory next door.

Both the furniture factory and an adjacent blinds and awnings factory have been destroyed.

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.

Firefighters used water jets to target fire pockets in the rubble. ( Twitter: Fire and Rescue NSW )

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.

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

Power is also being turned back on in affected streets, he said.

Superintendent Ticehurst said firefighters saved some pet fish from an adjoining factory that had suffered some minor fire damage.

"They had some fish in there that weren't going to survive with the power off, so the firies were able to go in there and get them out and bring them to the owner."

A partial wall collapse at the wrecking yard contributed to the fire spreading. ( Twitter: Fire and Rescue NSW )

He said firefighters had temporarily relocated the fish, which had been in an aquarium, into buckets to get them out safely.

The cause of the blaze is unknown.

Brunker Road was partially closed and caused traffic delays for several hours. It has now reopened.