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
Babies whose mother is immune will have some protection if they are currently being breastfed.
For children who are too young to have had both MMRs or who cannot be immunised for other reasons, the best way to protect them is to ensure everyone around them has been vaccinated – if you can't get it, you can't pass it on.
"If you think you may have been exposed to measles or have symptoms, please call your general practice first, 24/7," Dr Pink said.
"Calls made to general practices after hours will be answered by a nurse who will advise you what to do and where to go if you need to be seen.
"The MMR vaccine is very effective protection and we should see this as an opportunity for us all to make sure we are up to date with our vaccinations."
Measles is a serious and highly contagious viral disease where up to 30 per cent of those who catch it will develop complications.
Measles is spread through droplets in the air and through contact, so anyone unprotected who has been in the same room as someone with measles will likely get it.
Symptoms include; a respiratory type of illness with dry cough, runny nose, headache, temperature over 38.5C and feeling very unwell, and a red blotchy rash that starts on day 4-5 of the illness - usually on the face and moves to the chest and arms.