People in the South Island who haven't been immunised for measles are being warned an outbreak in the North Island is set to move south.
Nelson-Marlborough Medical Officer of Health Ed Kiddle said measles cases are increasing in the North Island, with 77 cases recently confirmed in Hamilton affecting five schools.
The outbreak started at Fraser High School and then spread to Raglan Area School, Hamilton Girls' High, Hamilton Junior High and Nga Taiatea Wharekura.
In total, there have been 210 confirmed cases in the North Island since December 2013, with at least 34 hospitalisations.
Dr Kiddle said most cases have involved people who aren't immunised, or are only partly immunised, particularly in the teenage and young adult age groups.
He said to have immunity, people must either have had measles or have received two doses of the MMR (measles, mumps and rubella) vaccine after the age of one.
Dr Kiddle said vaccination is free for anyone who wants it and being fully immunised is vital to avoid the disease which can have severe complications.
An immunologist believes the outbreak could be repeated elsewhere, as many people haven't been fully vaccinated and don't know it.
Immunisation Advisory Centre director Nikki Turner said while New Zealand now has high rates of vaccination in young children, that wasn't the case before the early 2000s.
To prevent further outbreaks, Dr Turner said education campaigns are needed to remind people to check their immunisation status. Those who aren't sure if they have had both their MMR vaccine shots should ask their GP.