Two new measles cases have been confirmed - one in Nelson and one in Christchurch - following an outbreak in March.
The cases are linked to three confirmed in Queenstown, Wanaka and Christchurch earlier this week after people were likely exposed to an infected person at Queenstown Airport on March 22.
The cases range in age from a 13-month-old baby to a 46-year-old, and Nelson Public Health staff are investigating whether the latest case had been immunised to protect against measles.
The cases outside of Nelson had not been immunised.
Canterbury DHB Medical Officer of Health Dr Ramon Pink says "it's important that people who think they may have symptoms of measles don't visit GP rooms or after-hours clinics.
"Instead, please phone your family doctor or general practice team first for advice, to limit further exposure to other people," Mr Pink says.
Public Health staff have been unable to trace the source of the outbreak, but the person may have had a relatively mild illness and could now be fully recovered, Mr Pink says.
"People are infectious from five days before the onset of the rash to five days after the rash starts and should stay in isolation during this time.
"This means staying home from school or work and having no contact with unimmunised people.
"If your vaccinations are up-to-date, you will have the best protection available. If you are unsure, you can check your vaccination status with your family doctor or general practice, although there is no harm in getting an additional dose."
The best way to protect yourself from measles is to have two MMR vaccinations.
The most common initial symptoms of measles are a dry cough, runny nose and high temperature. After four or five days, a rash will usually develop, spreading from the face to the chest and arms.
People with symptoms can call Heathline on 0800 611 116.