The number of measles cases in Auckland so far this year has hit almost 600.
The latest figures from the Auckland Regional Public Health Service show there have now been 596 cases - up 11 on yesterday's figures.
Nationally, the outbreak is the worst in two decades.
Doctor Maria Poynter from the Auckland Regional Public Health Service said the situation had been a growing concern throughout the past few months, and the figures would continue to balloon.
"There is enough measles virus circulating in the community, which means that people who aren't vaccinated and are exposed, they're really likely to come down with it."
The worst affected region in Auckland was the Counties Manukau DHB area, with more than 400 confirmed cases.
That figure represented just how infectious the disease was, taking hold in communities and skyrocketing amongst the unvaccinated, Dr Poynter said.
Counties Manukau was a vibrant and interactive community, which would have allowed the disease to easily spread, she said.
Demographics also played a part.
"We have a lot of pacific families and it's a relatively young population as well, both of which we are seeing increasing numbers compared to the total number of cases," Dr Poynter said.
Since the outbreak had started, Auckland Region Public Health Service had been working hard to contain the spread.
If people suspected they had been in contact with the measles, it was important to phone their doctor before visiting a clinic, Dr Poynter said.
Community-wide support for vaccinations was also a vital part of preventing the outbreak from worsening.
"The thing that we need to do now is have the community working together to increase vaccinations and I know that Counties Manukau DHB is working really hard to try and increase vaccination and find ways to provide more vaccination services to the population."
Gym members alerted
It also emerged today a person with measles had attended a gym in west Auckland, putting other members at the risk of getting the highly-contagious disease.
Les Mills New Lynn said in a social media post that the person had attended a class yesterday.
"This is part of the serious outbreak currently in the Auckland area," the post said.
The gym also urged others who went to the gym during the past weeks to contact their health professional if they suspect any illness.
"The Auckland Regional Public Health Service recommends that if you are concerned about measles you should call Healthline on 0800 611 116 or call your GP," the post said.
"Please do not just turn up to your GP, after hours or emergency department as you could potentially infect others."
As of midday yesterday, there have been 585 confirmed cases in Auckland this year.
Associate Health Minister Julie Anne Genter said she was concerned and had asked her ministry for urgent advice on how to tackle the outbreak.
- Measles is a highly-infectious viral illness spread by contact with respiratory secretions through coughing and sneezing.
- People are infectious from five days before the onset of the rash to five days after the rash starts.
- 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 believing they have been exposed to measles or exhibiting symptoms, should not go to the ED or after hours' clinic or general practitioner. Instead, call the GP first.