Aerial of Pukeuri Freezing Works. Photo by Stephen Jaquiery.
Fire and Emergency was called about 10.20pm to the Alliance Group site near State Highway One.
Southern communications shift manager Simon Lyford said three crews and a hazardous material command unit attended.
Firefighters found a cloud of ammonia on site and helped evacuate staff who were still in the building.
Lyford said police and ambulance officers also attended.