The Government has issued a health warning over imported frozen berries after linking them to Hepatitis A.
Four people have fallen ill across the North Island. Three of them were hospitalised but have since been discharged.
The Ministry for Primary Industries (MPI) says it was made aware a week ago and has since been working to pinpoint the source.
"We at MPI are putting in place a testing regime at the border for any imported berries coming in, and we'll also be testing stock that's already held in the country to make sure it's safe as well," says MPI's director of plants, food and environment Peter Thomson.
Fresh and frozen berries from New Zealand aren't a concern, but imported frozen strawberries, raspberries, blackberries, blueberries and boysenberries are, though so far MPI has been unable to identify the brand and origin.
"New Zealanders are real berry lovers, so we import around 7000 tonnes of berries a year from 25 different countries," says Mr Thomson. "So the information we've got to date hasn't enabled us to narrow things down."
Hepatitis A is a viral disease that affects the liver. It can take two to seven weeks to show symptoms, which can include fever, nausea and jaundice.
Retailers say consumers shouldn't be alarmed.
"The increased testing put in place by MPI is precautionary," says Retail New Zealand's Greg Harford. "It's designed to provide assurance that the products are safe. Most retailers already have established safety programmes in place."
But to eliminate the risk, MPI recommends the elderly and those with chronic liver damage should avoid frozen berries, and consumers, cafés and juice bars should boil or cook the fruit before consumption.