UPDATED 8.20pm: A fifth person has been diagnosed with Hepatitis A after eating frozen berries.

MPI spokesman Peter Thomson says an extensive investigation is underway in relation to frozen berries, and an increased surveillance programme has been introduced - including testing imported frozen berries at the border.

LISTEN: MPI spokesman Peter Thomson speaks with Larry Williams

"MPI was alerted to the case yesterday evening, following an investigation by the Ministry of Health and Environmental Science and Research (ESR)," Thomson said.

"In the new case, the person ate the berries in late October and reported symptoms at the beginning of this month. The person was briefly hospitalised for observation, but released the same day."

The infected person ate the berries in late October and reported symptoms at the beginning of this month.

Last week, FSL Foods directed by Ministry of primary industries, recalled all of its Fruzio Mixed Berries products.

FSL foods owner Mike Glover confirmed the company had recalled all of the mixed berries products.

"We've recalled all the products on Friday and over the weekend.

"It should be all off the shelves now," he said.

He said it was not unexpected that there would be more cases of contamination.

The incubation period for Hepatitis A is 15 to 50 days.

Last week FSL foods recalled all its Fruzio Mixed Berries products.

Meanwhile supermarket giant Foodstuffs New Zealand, which operates New World and Pak 'n Save supermarkets across the country, have confirmed they're continuing to liaise with the Ministry of Primary Industries.

Foodstuffs New Zealand corporate PR director Antoinette Laird confirmed all affected products were removed from shelves last week.

"We continue to encourage customers to follow MPI's medical advice and if they have any health concerns to contact their GP or local healthcare provider.

"Customers should return any Fruzio frozen berries to the store they purchased it from to receive a full refund," she said.

Progressive, which operates Countdown supermarkets, didn't stock any Fruzio mixed berry products, a spokeswoman said.

Caution urged following recall

MPI have advised anyone eating other imported frozen berries to wash their hands before eating and preparing food.

Anyone concerned should briefly boil any imported frozen berries before eating them, or ensure cooking exceeds 85 degrees Celsius for one minute.

"Elderly persons and those with chronic liver damage should avoid imported frozen berries that have not been heat treated."

People concerned about a potential risk to your health, or the health of others, should seek advice from their medical practitioner, or call Healthline on 0800 61 11 16.

People concerned about the safety of food products should contact the MPI consumer helpline on 0800 00 83 33.