The stench that came over Wanganui on Wednesday afternoon came from the Affco meatworks' blood-processing area.
Affco operations manager Troy Lambly said the plant had been shut down for maintenance since last Friday, and the smell came from an issue with restarting the blood processing.
"We have storage tanks, which are temperature-controlled," Mr Lambly said. "We didn't anticipate there being an issue in terms of restarting ... it appeared there was an issue there."
Horizons Regional Council sent someone to investigate after receiving "a number of calls" to their pollution hotline for a smell described as everything from "a bad fart" to "vomit" and "silage".
"An officer was sent to perform an odour assessment and the cause of the smell was attributed to the Affco Imlay plant," group manager strategy and regulation Nic Peet said.