Pacific Island children are among the most vaccinated in New Zealand, health authorities say, as they continue to try to stem the outbreak of measles in Auckland.

But it's likely environmental factors - including housing conditions and people moving frequently - have contributed to a disproportionate number of Pacific Island people catching the highly contagious disease.

There have been 944 cases of measles confirmed in Auckland, up 25 from Saturday, Auckland's medical officer of public health Dr William Rainger said today.

Of those cases, 650 had been in the Counties Manukau area, with children below the age of 5 and Pacific peoples disproportionately affected.

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Nationally there had been 1131 cases.

Rainger said there were about 20 new cases a day and it was still unclear when the outbreak would peak - though it was hoped it would be within weeks.

The high levels of vaccination currently occurring would help bring the outbreak to a conclusion, he said.

Counties Manukau DHB spokeswoman Carmel Ellis said uptake of MMR was very high for Pacific peoples, according to the National Immunisation Register.

There was a "slight blip" in the rate of vaccinations after two children died in Samoa following their MMR shots, Ellis said.

It was later found a nurse had accidentally mixed their MMR vaccine powder with expired muscle relaxant instead of water.

Ellis said confidence in vaccines had been rebuilt following that case. The main issue was around over-15s who were unsure of their vaccination status.

About three per cent of parents in Counties Manukau declined to have their children vaccinated, she said.

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Rainger said housing conditions and people moving frequently could be among the factors that had seen measles affect South Auckland so disproportionately.

It was hard to model how the outbreak would pan out as there was so much uncertainty around vaccination levels for over-15s. "It's not an exact science."

Rainger was still concerned by the outbreak but said he was encouraged by the "huge amount of vaccinations going on". There were 54,000 doses of MMR vaccine distributed nationally last week; 35,000 of those were in Auckland.

In the last 24 hours the Ministry of Health and Pharmac have done a stocktake of the vaccine.

"We know there's a lot of vaccine out there because of those huge numbers distributed last week," Rainger said. The key was to work out which clinics had run out or were on the verge of doing so.

The results of that stocktake would be known later today.

Rainger was asked if he felt there was enough warning that an outbreak was coming. He said given the last two outbreaks were in 2014 and 2011, "we've known there was the likelihood of outbreaks of some size every few years".

However the magnitude of this event could not be predicted because there was so much uncertainty about vaccination levels for over-15s.

He said it was the international situation that had made the current outbreak so unpredictable, as new cases of measles kept arriving in the country.

He compared New Zealand to an area of very dry bush where people kept throwing in sparks that caused fires to flare up.

Counties Manukau DHB was confident that its drop-in clinics had sufficient supply of vaccines but there was still work to do on whether GPs had enough, Ellis said.

Pharmac has also advised another 52,000 doses were due to arrive in NZ later in the week.

Ellis said there had been well over 4000 MMR vaccines given to date.

Three secondary schools in South Auckland's most affected areas have started immunising in schools and more are expected this week once equipment is in place.

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A seven-month-old baby with measles. Photo / Supplied
A seven-month-old baby with measles. Photo / Supplied