Three people are recovering after being struck by measles.
It is believed they were exposed to the highly contagious disease at Queenstown Airport.
Southern Medical Officer of Health Dr Marion Poore says none of the three victims were immunised.
"We've advised all local practices about the cases, and also asked them to be on the alert for possible new cases."
The cases were reported to health officials in Queenstown, Wanaka and Christchurch.
"It's possible that this is somebody that's travelled in from overseas with a relatively mild illness. Because measles is so infectious, they've been able to spread it to people who are unimmunised," said Dr Poore.
People are being urged to get their vaccination shots.
Canterbury DHB Medical Officer of Health Dr Ramon Pink says those experiencing the symptoms of measles shouldn't go to their GP or an after-hours clinic to avoid spreading the illness, and should call a medical professional for advice.
"People are infectious from five days before the onset of the rash to five days after the rash starts and should stay in isolation during this time."
The most common initial symptoms of measles are a dry cough, runny nose and high temperature. After four or five days a rash will usually develop, spreading from the face to the chest and arms.