A chemical hose has burst at Westland Milk Products factory in Hokitika, resulting in an uncontrolled chemical release.
Liquid was spilling out for two hours - and nitric acid was in the hot water vapour being released for a period of time.
The company's warning anyone who was on the Hokitika riverbank cycleway, behind the factory, between 1pm and 4.30pm to seek medical attention in case they came into contact with the potentially harmful liquid.
Communications manager Steve Attwood says the Grey Base Hospital and Hokitika medical centres were notified.
Anyone who came into contact with the liquid vapour may experience skin rashes, sore or damaged eyes or respiratory irritation.
Fire and Emergency and Police were unaware of the incident.