Investigations by WorkSafe found workers were aware of the open latch to the blade but management was not, due to its obscured location.
Health and safety matters at the site were raised in an ad-hoc way and staff were using work-arounds that managers were also unaware of.
WorkSafe's national manager of investigations, Hayden Mander, said the injury could have been avoided if the machinery was properly guarded to industry standard.
"Although a business might have standard operating procedures for machinery while it's in use, it's critical to think about how that extends to cleaning and maintenance too.
"Those uses can't be dismissed as out of sight and out of mind."
Mander said Mainland Poultry had now improved its health and safety procedures and their experience provided a timely warning for other businesses.
"Clear guidance and standards have been in place for many years, and the wider manufacturing industry needs to take notice to stop injuring and killing its workers."