The milk silo appears to have failed and collapsed, damaging a pipe bridge which caused the loud noise heard around the site, Fonterra's chief operating officer Robert Spurway said in a statement.
He added that while the cause of the incident is not yet known, there was no explosion and all staff were accounted for.
He also said: “Whilst some of the damage may take several weeks to repair, the situation is under control and we are now working to get the plant back up and running as normal.”
The collapse tore open the plant's 250,000-litre D4 milk silo, leaking milk across the site, knocking down part of the building and rupturing a steam pipe.
The milk spill has been contained.
Operations stopped
Normal operations at the plant have stopped, but essential staff stayed on to help turn off power and assist emergency services.
Fire services were working with police, ambulance and Fonterra staff to ensure safety in the affected area.
Edendale has been operating for more than 133 years and is the country’s oldest manufacturing plant.