An ambulance was called and paramedics are currently treating him on-site.
Company bosses have now closed the plant, which processes milk and cream, which is largely then exported to China.
"Our priorities at the moment are obviously with the gentleman concerned and making sure he's ok," the spokesman said.
"And the second priority for us is to then immediately establish how and why this happened.
"We've got a pretty bloody good safety record at Westland and we take it very seriously. Anything like this is automatically investigated so we can see if there's any lessons to be learned."
A St John spokesman said a 55-year-old male is being taken to Christchurch Hospital's emergency department in a serious condition.