Auckland health authorities hope they can control the spread of measles, following an outbreak at a North Shore school.
The virus has hit Westlake Boys High School, infecting 19 students and one staff member. That pushes the total confirmed cases to 40 this year.
Most cases have come from people being in contact with others who contracted measles overseas.
The Medical Officer of Health for Auckland's Public Health Service, Richard Hoskins, says they were not able to prevent the outbreak.
"Unfortunately, at Westlake Boys' High School the one case that came in was at school for several days while able to spread the disease.
"We didn't have a chance to control it before that because we didn't know about the case and so what we are hoping is that we can control it now with our public health measures."
Richard Hoskins says it is vital Aucklanders are immunised.
More than 750 people exposed to measles have been traced, investigated, and managed so far by the Public Health Service.
The last time Auckland had a measles outbreak, in 2011 and 2012, there were 500 cases with more than 80 people hospitalised.