A blaze inside a meat processing factory that had been burning for the past two days in Sydney's south has now been extinguished, NSW fire authorities have said.

More than 100 firefighters were involved in battling the fire since it engulfed the Haverick Meats factory in Banksmeadow on Saturday morning, forcing 40 workers to be evacuated.

About 30 firefighters remained at the scene overnight.

Superintendent Paul Bailey from Fire Rescue New South Wales said crews faced a difficult task in putting out the fire.

"We've been using demolition crews to remove the panelling and a lot of the internal contents of that building to allow us access into that building safely," he said.

"As you can understand it's a very dangerous and difficult scenario, with the roof collapsing and the potential collapse of the walls.

"The firefighters did a great job getting that fire out."

Superintendent Bailey said crews had now left the scene and it was up to the building's owners to demolish the factory.

"There is still a little bit of smouldering deep under other debris," he said.

"Residents should be aware there will be a small amount of smoke in the area."

A forensic investigation is yet to confirm the cause of the fire, but authorities are not treating it as suspicious.

"Our advice is that the fire started in a compressor that is adjacent to one of the cool rooms," Superintendent Bailey said.