More than 100 people may have been exposed to measles in the South Island.
Along with confirmations in Queenstown, Wanaka and Nelson, there are two in Christchurch - the age ranges for those infected are 13 months to 46 years old - and infected patients have likely visited supermarkets, restaurants and campgrounds.
Canterbury Medical Officer of Health Dr Ramon Pink told Newshub the hunt is on for patient zero, trying to work out who brought the disease in.
"Because we don't know where the source of measles is, who the individual was that was infectious, we don't know where they may have travelled," he said.
The highly contagious virus first showed up at Queenstown Airport late last month.
Dr Pink said it is important that people who think they may have symptoms of measles not to 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," he said.
Unimmunised people who have been exposed to any of the three cases are most likely to become ill between April 10 and 20.
They will first develop a respiratory-type of illness with dry cough, runny nose, temperature over 38.5C, and feel very unwell. A rash starts on day four to five of the illness, usually on the face and moving down to the chest and arms.
People are considered immune if they have received two doses of measles, mumps, rubella vaccine, have had a measles illness previously, or were born before 1969.
The best way to protect yourself from measles is to have two MMR vaccinations.