Public health officials are warning all schools in Hamilton to be on the alert as the number of measles cases in the city jumps to 77.
The outbreak, which began at Fraser High School, has spread to five schools.
One case each has been confirmed at Raglan Area School, Hamilton Girls' High School, Hamilton Junior High School and Nga Taiatea Wharekura.
Medical Officer of Health Anita Bell said information about the rise in cases has been sent to all schools, preschools and general practices.
Dr Bell says anyone under 45 who has not had two doses of the MMR vaccine is regarded as not immune.