There are five cases of paratyphoid in Hawke's Bay and a sixth in Auckland, Hawke's Bay District Health Board says.
The number of confirmed cases in Hawke's Bay Hospital rose from four yesterday.
All the patients are recovering.
At least three of the victims are known to have eaten mussels gathered from Napier's Ahuriri area, and the DHB is continuing to investigate other cases.
Medical officer of health Nick Jones said paratyphoid fever was a serious illness with fever, chills, headache, possibly a rash as well as severe vomiting and diarrhoea.
Without treatment, paratyphoid has a 10 to 15 percent mortality rate.
He said anyone feeling sick and who had eaten shellfish from the Napier Marina area should contact their family doctor or they could call HealthLine.