Rising concern that measles could be spreading unseen in communities around New Zealand

There are currently four active cases - in Auckland, Manawatu, and Nelson.

Three have no known link to overseas travel - leading health experts to believe the virus is transmitting undetected.

Auckland University of Technology's Dr Chris Puli'uvea says one person with measles can infect up to 18 others if they come into close contact.

He says the virus survives as droplets in the air for up to two hours, entering a new host through the nose or mouth.