Fourteen measles cases have now been confirmed in Canterbury, with the number likely to rise over the coming days and weeks.
Canterbury District Health Board said it can now be assumed that measles is circulating widely in the community.
The best protection is for people born after 1969 to have had two measles, mumps and rubella (MMR) vaccinations, the Board said.
Canterbury medical officer of health, Dr Ramon Pink, said that people in their late teens and early 20s are most at risk because a higher than usual proportion of that age group didn't have their scheduled MMR vaccinations, and because they are highly social and highly mobile.
"Anyone in this age band who isn't certain they have had both MMRs should contact their general practice team to arrange for an MMR vaccination – the vaccination and the appointment to have it is free," he said