Seventy-five firefighters have been instructed to wait, for their own safety, until a large fire burns down a factory in Sydney's south.

The firefighters, from 15 fire stations in the area, arrived at Haverick Meats on Green Street, Banksmeadow to try to control the blaze after it erupted about 8.30am on Saturday morning.

More than 40 workers evacuated the meat factory, which processed beef, lamb, pork and chicken for restaurant supply.

The fire started in an airconditioning unit and spread through the factory's refrigerated wall panelling.

Superintendent Ian Krimmer said the blaze was "very hard to control" because it was burning inside the insulated panelling, which will cause the building to cave in.

"It's now a matter of allowing the fire to burn until the building collapses," he said, adding this was likely to involve waiting overnight.

"We can't do anything except protect the surrounding buildings.

"Once it collapses and there's no risk of killing our firefighters, we'll pull the walls apart to extinguish the fire."

Black smoke clouds continued to billow from the roof of the meat factory on Saturday afternoon.

All workers were accounted for and no firefighters or workers have been injured.