Hawke's Bay District Health Board is investigating three cases of Paratyphoid which have been reported over the last week.
All three people have needed hospital care.
Paratyphoid is similar to typhoid fever and can cause serious illness.
Without treatment, it has a 10 to 15 percent mortality rate.
Hawke's Bay medical officer of health Oz Mansoor said one of the people affected reported eating shellfish gathered at Napier Marina.
He said there were signs warning against consuming shellfish there because of risks of contamination from sewage.
Dr Mansoor said anyone who had consumed shellfish from water that may be contaminated and was unwell should contact their family doctor.
Most cases of paratyphoid fever in New Zealand are associated with overseas travel.