There's no link to travel abroad and it is believed the children caught the disease locally. Photo: RNZ / Rob Dixon
Te Whatu Ora has confirmed one case and identified a second probable case, both involving young children.
The children had spent time in Auckland, Waikato and Tauranga visiting family members between 23 and 26 March.
There's no link to travel abroad and it is believed the children, who are related, caught the highly contagious disease locally, National Public Health Service clinical director Dr William Rainger said.
Rainger said about 20 to 30 people might have been affected so far, and urged people to keep up to date with vaccination.
"We are strongly encouraging everyone to be aware of their own immune status in regards to measles, but certainly to have their children vaccinated.
"It's free, and it's available from your healthcare provider."
He said anyone with symptoms should stay home and seek advice before visiting a doctor.
Symptoms of measles can start about a week to 18 days after a person has been exposed to the virus, Te Whatu Ora advised on its website.
A fever, cough, runny nose, and sore red eyes are among the most common symptoms.
It can be followed by a red or dark pink rash on the face or behind the ears about three days after the first symptoms. White spots can also appear in the mouth.
The spots, while not itchy, can be blotchy and close to one another. This can last for more than a week.
It can leave children in bed for about five days but still unable to attend school for a couple of weeks.