Measles is a "serious concern" in Auckland and at least 150 people are at risk of already having caught the deadly disease.
Yesterday, one Aucklander was confirmed to have contracted the highly contagious disease.
Now, Auckland Regional Public Health Service say hundreds could be in danger.
The person infected attended Manurewa's Clendon Medical Centre on three occasions on February 19, 20 and 21 and was assessed at Middlemore emergency department on February 21 at about 5pm.
ARPHS medical officer of health Dr Jay Harrower told the "Herald" they were in the process of contacting everyone who visited the medical centre or Middlemore Hospital's emergency department at those times and might have been exposed.
"The medical centre and emergency department have given us a list of about 150 people," Harrower said.
He stressed not everyone who might have been exposed to the disease would be infected.
People who were aged over 50 or had had the two vaccine doses at the age of 15 months and 4 years old were generally safe, Harrower said.
And, while the 150 people on the list were the number one priority, people they had been in contact with could also be at risk.
"We are phoning those people, seeing if they have contracted the disease and then finding out who they have been in contact with," Harrower said.
He said those 150 people could have also visited the medical centre and hospital with family or friends who would also need to be checked out.
It takes seven days after being exposed to the measles for a person to become infected.
"Seven days after exposure the person needs to be in quarantine until 14 days, even longer if they are still infected," Harrower said.
Given it had been more than seven days since the Aucklander infected had first presented at the medical centre, the outbreak was a serious concern, Harrower said.
He said it highlighted a desperate need to increase the vaccination coverage across the country.
The measles, mumps, rubella (MMR) vaccine, which protects against measles, should be received at 15 months and 4 years, according to the Ministry of Health's national immunisation schedule.
Earlier today, a Rangiora pre-schooler was the latest diagnosis of the measles in the country. Five cases have been confirmed - four are from Rangiora and one from Auckland.
Canterbury's public health medical officer Ramon Pink said measles was a "serious, potentially life-threatening disease" which had no cure.
"It is extremely infectious and is spread easily through tiny droplets a cough or sneeze ...
as the numbers of confirmed cases climb, the risk of getting measles increases for those not immunised."
Anyone with measles symptoms, or who believes they may have been exposed, can contact their GP at any time of the day or night for additional advice. Calls made after hours will be answered by a nurse.
• More information about measles is available at https://www.health.govt.nz/your-health/conditions-and-treatments/diseases-and-illnesses/measles
Symptoms of measles
• A respiratory type of illness with dry cough, runny nose, headache.
• Temperature over 38.5 C and feeling very unwell.
• A red blotchy rash starts on day 4-5 of the illness usually on the face and moves to the chest and arms.
• People are infectious from seven days before the onset of the rash to seven days after the rash starts.
Advice for people concerned
• Infected persons should stay in isolation – staying home from school or work - during this time.
• The best protection from measles is to have two MMR vaccinations. MMR is available from your family practice and is free to eligible persons.
• People are considered immune if they have received two doses of MMR vaccine, have had a measles illness previously, or were born before 1969.
• Anyone who thinks they have been exposed to measles or is exhibiting symptoms, should not go to the ED or after hours' clinic or general practitioner. Instead, call your GP any time, 24/7 for free health advice.