More than 30 Metropolitan Fire Brigade firefighters were called to a metal treatment factory in Murphy Street, Richmond, at 11pm on Wednesday, after a chemical spill produced a dangerous hydrogen-chloride cloud.

Police blocked off River and Burnley streets from Bridge Road, and nearby factory workers were evacuated.

Ambulance crews checked four factory employees, but no one was taken to hospital.

Firefighters wearing encapsulated gas suits entered to neutralise the cloud, containing the incident by 2.30pm.

Fire brigade Commander Barry Gray said employees had been evacuated by the time emergency serviced had arrived.

"Without the evacuation, it is likely employees could have been seriously affected by the chlorine,'' he said.

WorkSafe is investigating.