Auckland Regional Public Health Service says an outbreak of measles in Auckland could lead to a nationwide epidemic.
There are now five confirmed cases and two probable cases of measles in the city.
Clinical director Dr Julia Peters says it is extremely likely the infection will spread throughout the community. She says for every measles case, up to 13 other people could become infected.
She says even minimal contact can be a risk for anyone who is not immune.
Dr Peters told Morning Report health officials have been in touch with the parents and caregivers of children who may have been in contact with infected people. If the children have not been immunised against measles, she says they need to be kept away from school for 14 days.
The Ministry of Health says the current stock of vaccine supplies is enough to meet the normal immunisation schedule and negotiations are underway for extra doses.
Measles vaccine extended in Canterbury
The Canterbury District Health Board is offering the measles vaccination to younger children, in an attempt to combat the outbreak in the region.
Children normally receive the measles, mumps and rubella immunisation at 15 months and again at four years. But children can now be vaccinated at 12 months and three years.
The DHB has asked general practitioners to arrange for all children who are over one year old and have not been fully immunised to receive the vaccine.
Adults born after 1969 who have not been immunised are also being advised to have the vaccine. The DHB says it is likely people born before then have immunity from exposure to the disease.