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Pestell's Rai Bacon Company's ham products were recalled from stores across the South Island and in Wellington last month following fears they were contaminated.
The Ministry for Primary Industries said investigators traced the bug back to Pestell's and the processing environment, confirming it was the source of the contaminated product.
The December recall prompted the company to cease producing and selling ready-to-eat ham products after testing by a customer identified listeria in products.
The affected packaged ham products were specific batches of Pestell's Champagne Ham, cooked ham on the bone, boneless ham, boneless ham pieces, and sliced ham off the bone.
They are sold at some New World, Pak'n Save and Four Square supermarkets in the South Island, as well as other retailers nationwide.
NZ Food Safety national manager food compliance services Melinda Sando said: "Listeriosis infection can be serious among vulnerable groups such as pregnant women and their unborn babies, newborn babies, people with weakened immune systems and elderly people.
"For those in the high-risk groups, listeriosis typically has an incubation period of two to three weeks [or longer] before symptoms appear."
Healthy adults are likely to experience only mild infection, causing mild diarrhoea and flu-like symptoms, she said.