Auckland's public health service is warning a mumps outbreak could go on for some time unless people get fully vaccinated.
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Health authorities say those most at risk are aged between 10 and 29 and are part of a so-called lost generation who missed outduring a discredited controversy over vaccinations in 1989.
Auckland Regional Public Health Service has been told of over 450 cases this year.
That was up from 300 cases on 5 September, a jump of 150 in three and a half weeks.
"That's increasing on a daily basis," its clinical director Dr Julia Peters said.
"There's no evidence of things slowing down. It's a bit too early to say whether we've reached the peak."
The outbreak first started in West Auckland however Dr Peters said that had now spread across the whole city with it focused in Counties Manukau and West Auckland.
"There are a lot of schools that are impacted but it's also spreading within families and throughout the community."
She said Pacifika were disproportionately affected.
"Pacific people are making up around 50 percent of the cases."
Dr Peters said it was being driven by people who were not fully immune to the virus with 80 percent of people affected not vaccinated.
"So the only way we're going to stop this now is by increasing the levels of immunity across Auckland and in particular that target group.
"Earlier in the year, we really were trying to control it to see if we could bring it to a halt by our normal public health mechanisms.
"That - not surprisingly given the low rates of immunity, particularly with that 10 to 24-year-old age range - has proven not to be effective.
"So we're now directing our efforts into promoting vaccination."
Dr Peters said DHB colleagues, including Pacific health teams, were being talked to and schools as well as other primary health organisations had been warned.
"This outbreak, if we don't really get on top of vaccinating people, this outbreak is likely to go on for some time yet."
Students separated in exam season
A South Auckland school has had to separate some unvaccinated students during exams, for fear of spreading mumps.
Papatoetoe High School principal Vaughan Couillault said it had two students catch the virus, including one last week.
"[It's the] middle of exam season obviously, the end of term three preliminary exams, so we've had to take students who haven't been able to prove that they have been immunised and put them in a separate exam space ... so there has been lots we've had to do."
Mangere College had its first case of mumps this week with those close to the student asked to stay home if they were not vaccinated.
College principal Tom Webb said it had come in time for school holidays.
"That's come at a pretty good time for us, because that case was identified on Tuesday and it will mean students won't need to miss too much school.
"They're obviously not in as close contact with each other during the school holidays. Hopefully that will help us out with not getting any further cases."
Other schools in the area have sent letters home notifying parents about the risks.