An explosion at Fonterra's Edendale factory yesterday appears to have been caused by a chemical being poured into the wrong tank.
Five Fonterra staff overcome by fumes after an explosion and chemical leak at the plant were taken to Southland Hospital in Invercargill but were expected to be released last night.
An Invercargill Fire Service spokesman said it appeared a delivery company had put a chemical into the wrong tank.
It had reacted with residual chemicals in the tank and caused a "minor reaction" which blew the lid off the tank.
The injured were exposed to reflux s800, a chlorinated sanitiser which can cause breathing difficulties, anxiety and, in severe cases, burns or death.
The alarm was raised about 10.15am.
Chlorine gas was released into the air and several nearby workers were overcome by fumes.
Five staff were taken to hospital and treated for inhalation, while another six workers were treated by ambulance staff at the site.
For about two hours, traffic was prevented from going within 2km of the factory, which is on State Highway 1, 40km northeast of Invercargill.
Emergency services gave the all-clear to drivers about 12.15pm.
A Fonterra spokeswoman said most of the plant continued to operate during the incident and the affected area was back to normal by early afternoon.
The company and the Department of Labour are conducting separate investigations into the incident.
The Edendale Volunteer Fire Brigade was first on the scene and helped treat about 30 affected staff with a decontamination shower.
Other crew members worked on removing about 500 litres of the chemical and preventing it from entering nearby waterways.
The factory employs about 500 staff.
It is the second significant incident at the plant in the past four weeks.
A teenage construction worker was crushed by a concrete slab while working in a trench at the factory on December 22.
He was taken to Southland Hospital with serious chest injuries.
