The head of Starship Children's Hospital is warning some children are likely to die in the measles outbreak.

Photo: RNZ / Claire Eastham-Farrelly

The latest national figure for measles is 937 - the worst outbreak in 22 years.

Auckland accounts for close to 800 of the cases.

In an open letter in the Herald on Sunday, Mike Shepherd said children with serious medical conditions such as cancer or heart transplants could easily die from the measles infection.

He said the hospital sees critically unwell children with measles, pneumonia and brain infections caused by measles on their wards, and in intensive care.

Dr Shepherd said it's difficult to see children suffering from an entirely preventable disease, and is urging people to get themselves and their families vaccinated.

He said no one should assume they are immune, no matter their age.

Prime Minister Jacinda Ardern is also urging people to get immunised against measles.

Speaking at the launch of the government's new national cancer treatment plan, Ms Ardern said the strong advice is for anyone not immunised, to get vaccinated.

"I cannot state this more clearly, the best thing we can do right now is make sure that people are immunised.

"I think this is probably one of the issues that globally we are contending with and that is a movement of people who seek to try and convince people that they shouldn't be immunised."

Yesterday the Auckland Regional Public Health Service (ARPHS) announced New Zealand had now lost herd immunity against measles.

The incline of the epidemic curve is still steeply upwards, ARPHS medical officer William Rainger said.

"We haven't seen any sign of plateau at this stage. When we reach the peak we can't predict," he said.

He said although it was not the worst outbreak of measles in New Zealand - there was a 1997 epidemic that reached into the thousands - it had "truly exceeded the most recent significant event" of 2014.

"When you've got measles, you're infectious before you even realise it."

ARPHS was not advising people in Auckland to avoid public gatherings, however Dr Rainger said those who had or suspected they had measles should isolate themselves.

He said they were most concerned about children and those with weakened immune systems who are most at risk of complications from measles.

People are warned to watch out for symptoms of measles which include a fever, runny nose, cough and sore red eyes, with a rash following a few days later.

Anyone who thought they may have measles should phone ahead to their GP before going to the clinic, Dr Rainger said.