Workers at the Affco Rangiuru plant near Te Puke had to be evacuated after an ammonia leak.

Te Puke volunteer fire brigade chief officer Glenn Williams told the Bay of Plenty Times the leak was "isolated by contractors" and firefighters ventilated the room contaminated by ammonia.

An Affco worker, who did not want to be named, said the plant was evacuated about 12.45pm on Thursday after alarms sounded.

"We all went out thinking it was just a fire drill, they didn't tell us what the problem was."

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After spending 30 minutes outside the plant, workers were sent back inside "for a few minutes" to finish off "a few jobs", then sent on a 15-minute smoko break.

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.

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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.