The number of measles cases in Canterbury has gone up to 14, with health officials warning that number is likely to rise.
That's up from 10 just two days ago. The Canterbury District Health Board says it can be assumed measles is now circulating widely in the community.
People in their late teens and early twenties are most at risk because they're highly social and mobile and a higher proportion of that age group didn't get their scheduled vaccines.
Measles is a disease that's preventable, but it is highly infectious and potentially life-threatening.
Medical Officer of Health Dr Ramon Pink said as the numbers of confirmed cases climb, the risk of getting measles increases for those not immunised.
"People should stay in isolation from the time that they may have become infected until five days after the rash first appears," he said.
"This means staying home from school or work and having no contact with unimmunised people. If others in your household are unimmunised, they need to stay in isolation too for fourteen days from their likely first exposure."
Because measles is so infectious, it’s important people with symptoms don’t visit their GP or after hours clinics but phone their family doctor or general practice team for advice instead, to avoid infecting other people, he said.
Experts say the major problem in New Zealand is a fifth of teenagers and young adults not having the full immunisation dosage.
"Immunisation is highly effective. If you have enough of the population vaccinated measles cannot spread," Nikki Turner of the University of Auckland said.
And if you're not sure if you're immunised, the advice is to get a jab anyway.
The measles vaccine and appointment is free to all those who are eligible for funded healthcare in New Zealand.