The Ministry of Health is warning people travelling to Auckland to make sure they're vaccinated against measles.
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The outbreak has spread much more quickly in the last fortnight. About a third of all cases have come in the last two weeks.
Numbers outside Auckland have been fairly low recently; other regions have had just 10 confirmed cases in the last two weeks, compared to 230 in Auckland.
Schools have had to send hundreds of students home and this morning students at Manurewa High School weren't allowed in the gates without proof of immunisation, after 13 confirmed cases.
It followed updated figures from the government yesterday showing another 76 cases since last Friday - on top of the 132 since the Friday before - increasing the total of confirmed cases in New Zealand this year to 849.
It is the New Zealand's worst measles outbreak in 22 years.
More than 80 percent of all cases have been in Auckland and about two-thirds of those are in the Counties Manukau DHB, covering eastern and South Auckland.
Most regions haven't had a single case in the last couple of weeks - only Northland (two), Waikato (three), Bay of Plenty (three), Taranaki (two), Capital and Coast and the Southern DHB (one each) have had any since mid-August.
Dr William Rainger from Auckland Regional Public Health said infection numbers had been increasing quite dramatically in the last month or so.
"There's no end in sight in Auckland. It's continuing to increase. We can't predict when it will peak," he said.
Dr Rainger said family doctors had reported high demand for the MMR vaccine in the last few weeks, and that people appear to be heeding warnings to get vaccinated.
Researcher at Crown Research Insitute ESR Jill Sherwood said there was a high risk of the disease spreading to other regions because of two main factors.
"The first is that this will get much larger in the Auckland region. The second is that because of travel... we could start to see it spreading more in other parts of the country," Dr Sherwood said.
However, "immunity gaps" around the country - large enough portions of the population that are not immunised - mean some areas do not have "herd immunity" which is reached at a vaccination rate of 95 percent.
Otago University professor of public health Michael Baker said there was a risk Auckland's outbreak could spread.
"It's amongst the most infectious agents known," Dr Baker said.
"Because we are below that level [95 percent] the measles is so infectious it finds that gap. Wherever that gap exists in New Zealand it's going to find it. It's inevitable in other parts of New Zealand we'll see more measles cases until we can close that gap ... it will travel around the country as well.
"We have to get this uniformly high coverage across all these age groups to stop measles. It's a real challenge for our system. We're not far off but we're not there yet."
Dr Baker said there was quite a low number of people who chose not to vaccinate - most people simply had not got around to it - but it was a public health responsibility.
Young babies and people with immune deficiencies - such as cancer patients - cannot get vaccinated, and therefore are at a higher risk of getting measles. Dr Baker said there was a discussion to be had about making vaccinations compulsory.
Waikato health authorities confirmed this afternoon there had been three new confirmed cases of measles in the region in the past two weeks.
Medical officer of health Richard Hoskins said some cases were linked to the outbreak in South Auckland.
He said the outbreak in Waikato came either from people getting exposed to measles outside the region, or from exposure to local measles cases before they were diagnosed and isolated.
He said authorities were still working to identify all the people who may have been exposed and warned people in Waikato that the risk of further cases was high.
Prime Minister Jacinda Ardern urged anyone who had not been immunised against measles to get vaccinated.
"I'm worried about those who choose not to be vaccinated, it is the most important thing that we can do to prevent the spread of an outbreak like this so again we just encourage everyone to make sure that they are vaccinated."
In trying to stop the outbreak spreading further than Auckland, the Ministry of Health has issued a warning that people travelling to Auckland should get vaccinated first.
The Ministry said babies, who are particularly at risk, should be vaccinated at 12 months if they were in Auckland or travelling there. The vaccination should take place at least two weeks before travelling to allow immunity to develop.
Measles information:
- If you are showing symptoms, call a doctor but stay at home - do not visit the GP as you could spread the disease at the waiting room
- Measles symptoms include a fever, runny nose, sore red eyes and cough, followed a few days later by a rash usually starting on the face before moving down the body
- You can have measles and spread it to other people before you start to show any symptoms or feel sick
- It can take about two weeks to be fully immune after being vaccinated
- Healthline offers free advice and has a translator service 0800 611 116
- Measles has a 90 percent infection rate, meaning nine out of 10 non-immue people who come into contact with an infectious person contract the disease
- Measles is a serious disease, and can lead to complications including pneumonia and in rarer cases brain damage or even death
- Read this explainer from March for more information