Auckland Regional Public Health Service says the number of confirmed cases in the city now tallies 944, with 650 of those in South Auckland.

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Medical officer of health William Rainger said there were an average of 20 new cases of measles each day in Auckland.

Nationally there have been 1131 cases of measles.

Dr Rainger said the magnitude of this outbreak could not be predicted due to a lack of information about immunisation rates in people over 15 years old.

"I'm still concerned because this is a very significant outbreak of measles and measles is a significant illness. I'm encouraged because there is a huge amount of vaccination going on."

He said the most recent measles outbreaks in Auckland were in 2011 and 2014.

"We have known that there was the likelihood of outbreaks of some size every few years because of the international situation and because of vaccination coverage in New Zealand. What was not possible to predict was the magnitude of this event because of the lack of certainty of those people over the age of 15."

He said the scale of the outbreak was due to a number of factors, including housing conditions.

"It's the transmissibility of the disease and some of the conditions in South Auckland, along with vaccination coverage, all combine to create this situation."

The results of a nation wide stocktake of measles vaccines will be known later today, he said.

Counties Manukau District Health Board is training 25 nurses to give the measles vaccine to students at 34 high schools in South Auckland.

Its general manager child youth and family, Carmel Ellis, said school nurses have started giving the measles vaccine at three high schools in South Auckland.

"All of the schools we are going into are schools in areas where there are high numbers of cases [of measles]."

More than 20 marae hosting high school students for the largest school sporting event of the year are on high alert.

About 10,000 students from around the country are at the AIMS Games now underway in Tauranga, and many are being hosted by marae in the Bay of Plenty.

So far three teams had to pull out because of measles.

The managing director of a Māori health provider, Janice Kuka, said they had set up a strategy for marae to protect their families and prevent the disease spreading.

Measles information: