A contractor who poured chemicals into the wrong tank caused an explosion at Fonterra's Edendale factory in Southland on Monday morning, says Invercargill's Fire Service.
Six people were taken to hospital after the explosion, four of whom have been since discharged, while two others are still undergoing observation for breathing difficulties at Southland Hospital.
Invercargill Fire Service station officer Colin Russell says the chlorine-based chemical sanitiser Reflux S800 was poured into the wrong tank, blowing off its lid and sending a chemical cloud through the factory.
The accident happened just before 10.30am. The area was evacuated and fire crews from Wyndham, Edendale and Invercargill were called to the dairy co-operative's factory.
A Fonterra spokesperson said normal operations had resumed and an investigation into the cause of the accident was under way.
The accident closed both lanes of State Highway 1 at Edendale for a time.
About 30 staff and firefighters are making use of a decontamination unit to clear themselves of contact with the chemical.
Fonterra has begun an investigation.
Dairy Workers Union secretary James Ritchie wants an examination of the processes for ensuring contractors are properly trained in health and safety procedures on the site.
The Southland explosion was the third serious accident at one of Fonterra's plants in as many weeks - and the second at the Edendale plant.
In December, a teenage construction worker at the Edendale factory in Southland suffered serious injuries when a concrete slab fell on him.
An investigation is under way into the death of Theodorus Rangihu Blake, 52, who was crushed by a hydraulic press at Fonterra's Whareroa factory in Hawera, south Taranaki, on Saturday night.
Fonterra director Gary Romano says there is no room for complacency in its health and safety procedures