Waikato hospital patients might have been exposed to measles after a staff member was diagnosed with the disease.
The Waikato District Health Board said the person was working in the hospital while infectious, but before developing any symptoms.
It said it was now tracing all patients and other staff members who might have been exposed to the infected person.
The DHB said the staff member had been fully vaccinated against measles and this was a rare case of the immunisation not working.
It was the ninth case of measles to be confirmed in the region since 12 April, with another three under investigation.
Medical officer of health Felicity Dumble said anyone who was unsure if they had been vaccinated against measles should check with their GP.