Health authorities suspect measles may now be widespread in Northland and they're hearing only about the more serious cases.
Nine cases have been confirmed in the region, three of them since last week, and one of those people is in intensive care.
Medical Officer of Health Catherine Jackson says the new cases hadn't had any contact with the earlier ones, and that was concerning.
A third of the Northland cases have been admitted to hospital, and nationally the figure is close to 50 percent which is well above the admission rate in previous measles outbreaks.
Dr Jackson told RNZ Northland reporter Lois Williams the cases reaching hospital are the tip of the iceberg.