Dozens of firefighters from around Taranaki were called to a fire at a Fonterra plant in Eltham on Tuesday.
The Fire Service was alerted to an electrical fire in the plant's roof just before midday.
Communications spokesperson David Meikle said building was full of smoke.
Crews from Eltham, Kaponga, Stratford and Hawera are involved in the operation, along with a ladder and command unit from New Plymouth, about 50km.
About 550 people work at the Collingwood Street site, which is is the largest consumer and foodservice cheese manufacturing plant in Australasia.