Health authorities are investigating imported frozen berries after four people fell ill with the vomiting virus hepatitis A.
"Our investigations to date have not revealed a specific cause, but there is an association with consuming frozen berries," says Peter Thomson, of the Ministry for Primary Industries.
"Recent outbreaks in other countries also suggest this link."
In February, the ministry said it was monitoring frozen berry imports after a recall of frozen mixed berries in Australia prompted by cases of hepatitis A.
This evening, the ministry's director-general has issued a statement warning of a "potential risk associated with imported frozen berries" following the four cases "thought to be linked to packaged imported frozen berries".
Mr Thomson, the ministry's director of plants, food and environment, said the ministry was increasing surveillance of imported frozen berries, including starting a testing programme.
The ministry said: "Bringing frozen berries briefly to the boil will deal with any potential risk, as will cooking [that exceeds] 85 degrees Celsius for at least 1 minute. Washing frozen berries will not remove the risk."
Fresh berries were not implicated in the four cases of hepatitis A.
Nor is there evidence to suggest any link with frozen berries used in commercially-prepared and packaged products sold with a shelf-life claim.
Frozen berries are often used by cafes in smoothies and other ready-to-eat foods. The ministry says the risks associated with these items are the same as for home-made products and heat treatment will address any potential risk.
Hepatitis A virus is a waterborne and foodborne liver infection for which a vaccine is available. The disease is spread through contact with faeces of an infected person, such as through failure of an infected food handler to wash hands properly after using the toilet. After consuming the virus, it takes 15 to 50 days for symptoms to appear.
