There are now nine confirmed cases of measles in Northland, three more than at the end of last week.
The new cases range in age from 25 to 58, none of them immunised and all residents of Whangārei.
Tracing their contacts has identified people had unknowingly visited Whangārei Hospital when they were infectious.
For privacy reasons, the Northland District Health Board would not say if the new cases were visitors or patients.
Dr Catherine Jackson, Medical Officer of Health, said the cases highlight the extreme care everyone needs to take now that measles is circulating in Northland.
"Measles is a serious, highly infectious, potentially life-threatening disease, and immunisation is the only sure way to avoid getting measles.''
Symptoms start with a high fever, cough, runny nose and sore red eyes, then after three or four days a rash. It is infectious for five days before the rash shows until five days after, and spreads very easily.
All three new cases have been very unwell, with one person being cared for in ICU.
There are another 30 people in their close contacts in isolation because they are at high risk of getting measles and passing it on.
Isolation means staying home from work, school and social occasions.
If there is a case of measles at a school, early childhood centre or workplace, the Northland Public Health Unit (NPHU) will request anyone who is not immunised and may have come into contact with measles to stay away for two weeks. NPHU is aware of the impact of isolation for a week or more has on family and working lives, Jackson said.
"However, we have to balance this with the need to prevent further spread of measles, and to protect infants, those most at risk of complications if they contract the disease, and the wider community.
"We do want to thank people for staying in isolation and for being immunised because this is really helping stop the spread of measles."
A complicating factor at this time of year is the presence of influenza which has similar signs and symptoms at the start of measles infection, she said.
"If you are feeling unwell please don't visit family or friends in the hospital until you are feeling better.
''Some people get very unwell with measles, if you need to see a doctor please call ahead to your GP or the emergency department so they can prepare for your arrival.''