Health officials aren't sure how three people have contracted paratyphoid fever in Napier.
Most cases in New Zealand are due to overseas travel. It is spread through food or water that is contaminated with the faeces of infected people.
Hawke's Bay DHB Medical Officer of Health Dr Oz Mansoor says the recent cases are unusual as the people infected have not been overseas.
"We have three cases who did not have overseas exposure. Most of the cases we do get of paratyphoid, they're usually infected overseas and they bring it into New Zealand and it doesn't spread on," he told Newshub.
Paratyphoid is similar to typhoid fever and can cause serious illness. All three of the reported cases have required hospitalisation.
It has been confirmed one of the cases was due to shellfish collected near the Napier Marina being eaten. There are currently signs at the marina warning against eating shellfish collected in the area due to sewage contamination.
Hawke's Bay DHB is warning people to be careful about the collection of shellfish and to contact their family doctor if they have eaten shellfish and feel ill.