A fourth person in the Tauranga-Western Bay of Plenty area has been treated for listeria but the sources of the outbreak have yet to be found.

The outbreak has so far resulted in three hospitalisations and one death in Tauranga, health officials say.

Toi Te Ora Public Health Medical Officer of Health Dr Phil Shoemack said the fourth recent case reported in the Bay of Plenty involved a resident from the Western Bay of Plenty.

Shoemack said the fourth case was diagnosed last week when the person was admitted to hospital and they have since been discharged.

He declined to reveal the age or gender of the patient at this stage in the investigation.

Shoemack said the three earlier local cases involved people aged between 70 and 90 and the person who died was terminally ill.

The extent to which listeria contributed to their death was being investigated by Toi Te Ora, he said.

The first listeria case was diagnosed on July 12, the other two cases on July 20 and the fourth case was reported last week.

Since July 1, there has been one case reported from Auckland, four cases from Bay of Plenty and two cases from Wellington.

The Ministry of Primary Industries said there was no evidence of any link to the Tauranga cases.

There is also no evidence the cases are linked to the Talbot Forest Cheese products pulled off shelves late last month due to a potential unconfirmed listeria risk.

Shoemack said foods collected from the homes of confirmed cases were being tested to establish the source of contamination.

He said it was hoped the laboratory results would be available by the end of the week.

Shoemack said there was no evidence the Bay of Plenty listeria patients were connected to each other.

Ministry of Primary Industries director of compliance Gary Orr said the Ministry's investigation concluded there was no link between the three earlier Tauranga cases and the recalled Talbot Forest Cheese products.

"We are continuing to work alongside Toi Te Ora Public Health to identify the source of listeria in each of these cases," he said.

Orr said testing of cheese products and the processing environment continued as part of a number of actions to ensure there was no ongoing risk to consumers.

"Until all actions have been completed distribution of the cheeses remains on hold."

Listeria monocytogenes is a bacteria that is naturally present in the environment.

These cases have prompted a warning over food safety for the elderly, pregnant women, babies and people with compromised immunity.

Listeria symptoms vary but can include fever, headaches, tiredness, aches and pains, diarrhoea, nausea or stomach cramps.

Symptoms can occur between three and 70 days after eating contaminated food.

Foods to avoid for those in the at-risk group:

- Uncooked, smoked or ready-to-eat fish or seafood, including oysters, prawns, sushi

- Paté, hummus and tahini-based dips and spreads

- Cold pre-cooked chicken

- Processed meats including ham and all other chilled pre-cooked meat products
(including chicken, salami and other fermented or dried sausages)

- Pre-prepared, pre-packaged or stored salads (including fruit salads) and coleslaw

- Raw (unpasteurised) milk and any food that contains unpasteurised milk

- Soft-serve ice creams

- Soft, semi-soft or surface-ripened soft cheese (brie, camembert, feta, ricotta, roquefort)