Six ambulances have been sent to a factory in south-west Sydney to treat a number of people overcome by fumes from a chlorine spill, which has forced the evacuation of 25 workers.

Paramedics were called to the Ecolab chemical factory on Marigold St in Revesby at 8am (AEST) on Thursday, with reports people were suffering effects of chemical exposure such as vomiting, breathing difficulty and eye irritation.

The Ambulance Service of NSW has sent 12 paramedics in six ambulances to treat patients at the scene.

"Ambulance has established a staging area to treat the persons from inside the building," it said in a statement.

NSW Fire Brigades have evacuated 25 workers from the Ecolab factory because of the chlorine spill.

Three Hazmat vehicles and five fire trucks have been sent to the scene, a brigades spokeswoman told AAP.