In the midst of a measles outbreak, a large high school has discovered about a third of its students are not vaccinated.

Manurewa High School in south Auckland has a roll of about 2000 students, but a check of the National Immunisation Register last week revealed only about two thirds were immunised, principal Peter Jones said.

Last week it was revealed about 300 students had been sent home after 13 pupils were diagnosed with measles. 

Those 300 students were beginning to return, but the number of measles cases at the school had since risen to 14, Jones told Stuff.

Jones said the school was working with the Counties Manukau District Health Board to immunise students who were not vaccinated.

He said the number of unvaccinated students was higher than the school first thought. 

As a result, a free immunisation clinic had been set up on Tuesday to vaccinate students over 15.

There were also plans for a follow-up clinic to run for students aged under 15, Jones said.

Until they could be vaccinated, unimmunised students were being advised to stay away from school for a week.

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"It was our intention to get a better handle on this," Jones said.

"We have in the community quite a bit of transience and people coming in from overseas. We're trying to get more accurate data to know how to target students."

The number of confirmed cases of measles in the country has reached 937, with 804 of them in Auckland.

South Auckland has been hit particularly hard and more than 50 schools in the region have dealt with measles this year.

On Saturday, a guest with the highly contagious disease put hundreds of students at risk of contracting the illness at the St Peter's College ball.

Health officials are urging people to get vaccinated and the government activated a national response late last week.

The head of Auckland's Starship Children's Hospital warned some children were likely to die because of the outbreak.

People most affected by the outbreak have been children and those aged between 13 and 19.

Young babies were the most vulnerable, and the first MMR vaccine had been brought forward from 15 to 12 months in Auckland to try to address that.

Jones said the measles outbreak had had a "real" impact on students' learning. 

Mock exams were about two weeks away and Jones said it was "feasible" to push them back with the number of students absent.

Through an online learning platform, some students had been interacting with their teachers, work and resources, Jones said.

"It's a help to keep students connected, but it's not a substitute for being here," he said.

"Some students are proactive with it, other's aren't, and it depends on how they're feeling."

Jones said the school was hoping to have everyone back in class soon, despite warnings to expect further measles cases.

"Nobody realised across Auckland the scale of non-immunisation and if you did no one was talking about it," Jones said.

"We've had lots of support, help and advice, and we're very grateful to run this clinic. We want to make it easier to support whānau, the community and students.

"Our message to our school community is to follow the communications we're putting out and instructions and if anyone is not sure get in contact."