Auckland Regional Public Health says it appears an outbreak of measles in Christchurch has now spread to Auckland.
There have been three confirmed cases and three probable cases of measles in the city.
Clinical director Julia Peters says for every measles case up to 13 other people could become infected.
She says parents are being encouraged to immunise their children as the number of measles contacts increases.
Dr Peters says there is particular concern about any child under the age of five who has not been immunised because even minimal contact with a measles case can put them at risk.