Health officials are warning measles could kill as cases near 1000 across the country, including more than 800 in Auckland.

More than one-third of those have ended up in hospital, and the Auckland medical officer of health fears the outbreak will turn deadly.

At Starship Hospital, four children are seriously unwell with measles - one is critical.

"The fatality rate is about one in a thousand but that doesn't mean you have to reach a thousand before someone dies," Auckland medical officer of health Dr William Rainger said. "Someone could die at any time - it just means the more numbers, the more likely it is that there will be a fatality."

Measles cases nationally are up to 963, with 804 in Auckland and most of those are in the Counties Manukau region. They're climbing by almost 20 a day, and are set to continue.

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Despite the rising numbers, anti-vaccination messages are still being circulated. One group is offering a course which covers informed consent, vaccine components and alternative options.

The registration form asks parents, "What initially caused you to question vaccines?"

The group didn't respond to Newshub's requests for comment, but the measles message from health officials was clear.

"It can be fatal," Dr Rainger said. "It can leave people with serious and permanent brain injuries, and it can be prevented by MMR, those are the scientific facts."

Pop-up vaccination clinics are trying to slow the rate of transmission - already giving 300 extra vaccinations. Newly trained school nurses will also be offering MMR jabs at 34 south Auckland schools from next week - but there's no centralised register for children over 14-years-old.

"We're finding it really difficult to figure out how many people we need to immunise or how many people aren't fully immunised," said Carmel Ellis of the Counties Manukau District Health Board.

More than 50 schools have confirmed cases. Officials are now working with St Peter's College after someone with a suspected case attended a school ball on Saturday.