Three cases of measles have been confirmed in the last four days in the South Island.
The cases have been reported in Queenstown, Wanaka and Christchurch, a spokeswoman for the Canterbury DHB said.
At this stage it appeared the common place of exposure for all three cases was Queenstown Airport, where all three people were likely to have been in contact with an unknown infectious case on March 21 or 22.
"This person may have had a relatively mild illness and will now be fully recovered," the spokeswoman said.
Any other people exposed during the same time would now be at the end of their maximum incubation period and unlikely to get sick.
None of the three known cases were immunised for measles.
"Southern and Canterbury DHBs were working together on this investigation and close contacts of all three cases have been identified and are being followed up."
Unimmunised people who had been exposed to any of the three cases were most likely to become ill between April 10-20.
People are considered immune if they have received two doses of MMR (measles, mumps, rubella) vaccine, have had a measles illness previously, or were born before 1969.
Canterbury DHB medical officer of health Dr Ramon Pink said it was important people with symptoms don't visit GP rooms or after-hours clinics but phone their family doctor or general practice team first for advice, to limit further exposure to other people.
"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," Dr Pink said.
If people called their GP team after hours a nurse will answer the call and advise what to do and where to go if needed.
Measles is a highly infectious viral illness spread by contact with respiratory secretions through coughing and sneezing.
Unimmunised people exposed to measles first develop a respiratory type of illness with dry cough, runny nose, temperature over 38.5C, and feel very unwell.
The rash starts on day four to five of the illness, usually on the face and moving down to the chest and arms.
The best way to protect yourself from measles is to have two MMR vaccinations. MMR is available from your family practice and is free to eligible persons.