A man was injured at the Rangiuru Affco Talleys plant on Friday.
New Zealand Meat Workers Union director of organising Darien Fenton said according to workers who witnessed the accident, the worker was knocked out, suffered lacerations to his face and head and lost some teeth.
The accident took place on the Mutton Slaughter chain where the worker was caught up on a spreader, she said.
Affco general manager Andy Leonard said the company was investigating the incident.
Worksafe New Zealand was also investigating.