Fonterra says a chemical leak at a plant in Southland was caused by water washing back into cleaning tanks and overflowing.
Six fire teams were called to the Edendale plant early on Tuesday morning, where staff reported about 5000 litres of nitric acid and a caustic alkali pouring into a catchment area.
Fonterra southern operations manager Richard Gray says the plant experienced a "system error".
"We have worked with the fire service to contain the overflow and there is no risk to neighbours or our staff on site.
"The chemicals that overflowed were caustic and nitric, both of which are used to clean our tanks as they are easily neutralised and turned into basic, non-toxic salts."
Edendale Volunteer Fire Brigade's Scott Cameron says no one was injured. It took 25 firefighters eight hours to bring the situation under control.
"We go through a process to render the chemicals safe, they're contained in chemical bunds that were at Fonterra so it was all in a good safe state," he says.
"These sorts of events, they seem to feel like they go for a long time but there's a process that we go to ensure our safety and the safety of the workers on site. It's normal for a chemical incident to have a long duration."
Southland District Council is sending an investigator to the scene.
Newshub.
