The Waikato District Health Board expects a measles outbreak in the town of Te Awamutu to spread to surrounding towns next week.
The DHB says the number of cases has increased by one, to nine, since it was alerted to the outbreak on Thursday afternoon.
But it says many more cases are likely when students return to school on Monday and spread the disease.
Medical Officer of Health, Dell Hood, says those infected are teenagers who appear to have come in contact with measles at the end of last term, including some from Te Awamutu College.
Dr Hood cannot confirm yet if the initial source of the illness is linked to an outbreak in Auckland, where the number infected has reached 76.
An investigation is underway to determine the source of the illness, and how widespread it is.