The Affco worker said they were not told there was an ammonia leak until after they had been sent back inside the plant.
"All workers felt really unsafe."
The worker understood the ammonia had leaked from a freezer and had "affected the whole plant".
The workers were sent home for the rest of the day and were told to be at work at their normal time Friday, as the problem would be fixed.
Interim general manager of Affco New Zealand Nigel Stevens said ammonia leaked in the cool store where maintenance was being done.
"There has been no serious harm to anyone that we are aware of," said Stevens.
A Fire and Emergency spokeswoman said five fire crews were called at 1.03pm.
The leak was isolated and firefighters ventilated the plant.
Fire crews came from Maketu, Te Puke and Greerton and were still at the scene at 4pm.
The spokeswoman said the ammonia levels would be retested after ventilation.
A St John spokeswoman said an ambulance attended the evacuation as a precaution but no one was injured.