A Ministry of Primary Industries spokesman said 38 people had been admitted to hospital but it was still determining the source of the outbreak.
"The people who have become ill with Yersinia have been interviewed on what foods they have been eating and this information has been cross-referenced, giving us sources to investigate further."
The MPI's investigation was over a range of foods and person-to-person spread was uncommon, he said. The spokesman recommended normal food safety procedures. "If it looks off or smells off, throw it out. Otherwise keep fresh food chilled and wash fruit and vegetables before eating."
Meanwhile, supermarket owners say no complaints have been made to them.
Foodstuffs spokeswoman Antoinette Shallue said it had not had any confirmed Yersinia food-borne illness attributed to products sold in any of its New World and Pak n' Save stores.
A Countdown spokeswoman said all produce, including locally grown carrots and lettuce, were produced under strict quality assurance standards.
"MPI and the Ministry of Health are currently investigating an outbreak of illness that mostly happened in August and September and they haven't identified the specific cause or source as yet. Several produce retailers have been asked to help with the investigations, Countdown included.
"To date we haven't found any cause for concern from our products or had any complaints relating to this issue."
Neither spokeswoman would say whether there had been a notable drop in people buying bagged carrots and lettuce.
If people believe they may have contracted Yersinia, they are advised to contact their GP or call Healthline on 0800 611 116 for help.
