Scores of Auckland children not vaccinated against measles are being urged to stay away from school after coming into contact with the potentially fatal disease.

They will be kept at home for 14 days after their last contact.

Two children have been confirmed by laboratory results to have the highly infectious illness.

The youngsters mixed with hundreds of pupils from their own schools as well as 300 others at a competition at the North Harbour Gymnastics Centre in Hillcrest, and parents whose children attended that event are being sent letters.

Health officials say the stay-away is voluntary but they have the power to ban people.

The new cases bring to five the number of confirmed cases of measles in the region, after the outbreak of the viral infection spread north from Otago and Christchurch.

But despite the small number of cases, health authorities are taking no chances.

The Auckland Regional Public Health Service said one of the new cases is a pupil at Diocesan School for Girls in Epsom and the other is from Vauxhall School in Devonport.

As around a quarter of schoolchildren have no immunity to measles - because of not having had the two free jabs, or vaccine failure - the service expects many more cases.

Measles is so infectious that simply being in the same room as someone with it is considered to be "contact".

One person with measles typically infects up to 13 others. Swine flu is much less infectious: it is estimated that each case will infect two other people.

The age and gender of the Vauxhall pupil, who is in the school's combined Year 5-6 syndicate, have not been stated. Principal Aaron Kemp said the pupil was "fine".

The Herald understands the Diocesan girl is 12. Principal Heather McRae said she did not know the girl's condition.

The public health service's clinical director, Dr Julia Peters, said it was estimated the Dio girl had been in contact with around 300 school students - 86 were unvaccinated.

The Vauxhall pupil had been in contact with 96, but the number unvaccinated was not known.

"They attended North Harbour Gymnastics [Centre] and there were 300 contacts there, so we have sent the same letter to the parents and caregivers ... We do have the power to exclude non-immune students but at the moment we are using the approach of strongly advising them to stay away."

Forced exclusions would be used if people did not co-operate, but the experience with swine flu earlier in the pandemic was that people had stayed home when public health staff asked them to.

Dr Peters said 14 days away from school was a big disruption and she urged parents of unvaccinated children to arrange innoculations urgently.

Dr Nikki Turner, of Auckland University's Immunisation Advisory Centre, said nearly 70,000 children under the age of 5 were at risk of catching measles.

"Many New Zealanders may not have seen measles, or have forgotten that it can be a serious and sometimes fatal disease, even in healthy, well-nourished children."

* The illness

One person who catches the measles virus can infect up to 13 others.

80-85 per cent of children have been vaccinated against it by age 5.

90 per cent of those vaccinated are protected.

The disease incubation period is about 10 days.

Patients infectious for a day before symptoms start, until four days after rash breaks out.

1 death and 1 brain inflammation for every 1000 cases.