Public health officials in Auckland are trying to identify anyone who could have come in contact with a person infected with measles, to stop further transmission.
They say the person recently returned from overseas, and went to the emergency department at Auckland City Hospital, in Grafton, on Sunday evening (17 September).
The Auckland Regional Public Health Service (APRHS) said close contacts who were in the emergency department at the same time were being informed.
The person was also at the Bake and Beans bakery in Takanini for about 15 minutes last Tuesday, and people who were there between 5.15pm and 6.45pm were at risk of infection.
Anyone who developed symptoms should stay home and phone their doctor or Healthline on 0800 611 116, said clinical director Lavinia Perumai. They should not go to the doctor or hospital without calling in first.
Early symptoms include fever, cough, runny nose and sore and watery 'pink' eyes, followed by a blotchy rash.
Other contacts include their household members and some work colleagues. They were not infectious when they flew into New Zealand, ARPHS said.
The patient was being cared for in isolation at the hospital.
"Our first priority is to respond to this single case to stop further transmission," Perumal said.
She urged people to make sure they and their children were up to date with their vaccinations - which are free for everyone under 18, and for New Zealand residents 18 and over.
Measles is an incredible infectious disease, which Perumal said was "very serious".
"It is important that people travelling overseas make sure they have had their MMR so they do not bring the virus back into the country."
The latest case was Auckland's fourth this year, all linked to overseas travel.