Three people have been confirmed with measles and 51 are in home quarantine as health authorities in Auckland battle to stop the outbreak spreading.
A person with the disease who ignored instructions to go into isolation has potentially put hundreds of people at risk of catching the virus.
Instead of isolating themselves, the person roamed the SkyCity Casino and SugarTree apartment complex on Union St.
An alert on Wednesday said the person contracted measles on an international flight late last month.
Auckland Regional Public Health Service said yesterday a number of people had since come forward with measles symptoms.
Three people were known to have the virus while others have been quarantined for tests.
"Anyone who is instructed to go into isolation is there because they could potentially develop measles," Auckland medical officer of health Richard Hoskings said.
"The numbers in isolation change frequently as we get lab results showing if people are immune.
"Currently there are 51 people in isolation but this should decrease ... pending lab results."
A person will need to go into isolation for 10 days if they have measles, if they are a 'contact' and know they are not immune or if they are a 'contact' and unsure of their immunity.
"A contact is someone who has been exposed to the disease," Dr Hoskings said.
"If a contact is unsure of their measles immunity they need to phone their doctor or Auckland Regional Public Health Service so a health representative can visit their home to take a blood sample to determine immunity.
"At this stage a 'contact' must continue to stay at home and isolate themselves from other people."
If a person must go to the doctor they should phone in advance; if they have measles they could infect other people by being in the waiting room or simply walking past them.
When the results of the blood test are ready, they will be contacted by phone, Dr Hoskings added.
"If the blood test shows the 'contact' is immune to measles then they are not a risk to other people and will be given permission to leave isolation.
"If their blood tests show they are not immune they will be instructed to remain in isolation for a specified period of time."
The person believed responsible for the outbreak and who ignored advice to seek help flew to New Zealand on January 30 on China Southern Airlines flight CZ305 and was sitting two rows behind an infected passenger.
A decision is still to be made over the possibility of any action against the person.
Measles cannot be treated once contracted. Vaccinations offer protection. It is a serious illness so check your immunisation is up-to-date. The disease is highly infectious.
For advice call the Healthline on 0800 611 166.