Health officials have been alerted to meat containing listeria at three processing plants in Wellington and Porirua.

The bacteria is particularly dangerous to newborn babies, pregnant women, the elderly and people with weak immune systems.

Regional Public Health and the Primary Industries Ministry are investigating the processing plants after meat samples at two of the plants tested positive for listeria during a routine survey.

The other plant found listeria in its own testing and reported the results to health officials.

Regional Public Health would not identify the plants.

Only one case of a person contracting listeria has been reported to public health in the past six months. It was not linked to meat from the processing plants under investigation, Medical Officer of Health Margot McLean said.

"It's a reasonably rare infection. The cases that we are notified about are at the severe end of the spectrum."

A bout of listeria in a healthy person could result in fever, but if a vulnerable baby or adult contracted it the symptoms were much worse and included fever, vomiting, headache and signs of infection on the brain, Dr McLean said.

Listeria is found in soft cheeses, processed meat and raw milk. When these products are left at room temperature or refrigerated the bacteria multiplies.

The ministry was unable to questions by last night.

- © Fairfax NZ News