MPI said it was too early to be certain about the source of the pathogen and as a result direct people away from foods that have no proven risk for consumers.
However, MPI said there was a credible link that food was the likely source.
Overseas outbreaks of yersinia pseudotuberculosis had been linked to contaminated fresh vegetables and fruit, contaminated water and animal contact.
MPI said it was investigating a range of foods.
Chief medical officer of health Phil Shoemack said Toi Te Ora recorded four cases in the Lakes district and one from the coastal Bay.
"This is an unusual cause of food poisoning. That's why it sticks out - to have more than 100 cases around the country within two weeks is most unusual," he said.
"It appears to be related to consumption around carrots and packaged lettuces but we haven't managed to track an actual suspect yet."
Dr Shoemack said Toi Te Ora was working collaboratively with Wellington health officials in finding the exact cause behind the outbreak but there was still a lot of information unknown.
"We've spoken to each of the cases about what food they consumed in the days prior to getting ill and it appears the most likely risk factors are the carrots and lettuces which are pre-packaged.
"It appears loose fruit and vegetables are not involved.
"It's very much a day by day investigation and hopefully we are getting closer to nailing what it is."
Dr Shoemack said the issue was likely to be a batch problem, and if so it should dissipate soon.
For now, he reminded people to take extra care with personal hygiene when preparing and consuming food. Plus washing any raw fruit and vegetables thoroughly.
If anyone finds themselves suffering from similar symptoms, Dr Shoemack recommended they contact their GP straight away.
The first symptoms of Yersinia pseudotuberculosis include:
•abdominal pain (often on the lower right), which may mimic appendicitis
•fever
•sometimes diarrhoea.
About 1-3 weeks later, you may get a rash and joint pain. The joint pain can last up to 6 months.
In rare cases Yersinia pseudotuberculosis infection can lead to sepsis (infection in the blood). This is more common in people with weakened immune systems.
See your doctor, or call Healthline on 0800 611 116 for advice if you think you might have Yersinia pseudotuberculosis. There's a laboratory test which can check for the disease.
- additional reporting APNZ
