Health experts in Auckland have criticised anti-vaxxers as "misinformed" as the number of confirmed cases passes 800.
Measles should not be treated as a trivial illness, Auckland Regional Public Health Service (ARPHS) spokesman William Ranger said.
The number of confirmed cases has risen to 804 in Auckland, and Ranger said last week there was a child who was "very unwell" - but he was unsure whether anyone had been diagnosed as critical.
Auckland's Middlemore Hospital has opened a measles ward.
In response to anti-vaxxers, Ranger said they were "misinformed" and that people could be "assured" that the MMR vaccine was "very safe".
"It is not a trivial illness, it can be fatal."
Ranger also mentioned people mostly affected by the outbreak had been children and those in their 20s.
The fatality rate for measles was one in 1000, Ranger said and as the numbers rose the likelihood of a fatality did too.
Earlier on Monday, schools and a marae asked Auckland health officials for more pop-up vaccination clinics to help combat the measles outbreak.
More than 300 people were vaccinated over the weekend at free clinics set up by Counties Manukau District Health Board.
The clinics are a response to the worst outbreak in New Zealand in 22 years, with 937 cases reported nationwide, 700 of them in Auckland.
- Anti-vaxxers target new parents, while others nurse their critically ill kids
- Twenty new measles cases per day as Auckland's outbreak worsens
Health officials are urging people to get vaccinated and the government activated a national response late last week. The head of Auckland's Starship hospital warned some children were likely to die because of the outbreak.
More than 50 schools in the Auckland region have dealt with measles this year.
Mangere College had three confirmed cases a few weeks ago with about 100 students asked to stay home.
Principal Tom Webb said they had been gathering immunisation records for students and helping letting families know where they could go to get vaccinated.
"We've been publicising that information to all of our families, encouraging them to get vaccinated.
"One of our school nurses has just been on a training course on Thursday and Friday so that she's now qualified to do the vaccinations herself which means we'll be able to start vaccinating any students who haven't been vaccinated before.
He said the school would have to notify families first but it hoped to get started early this week.
The outbreak caused the cancellation of a high school rugby league championship over the weekend.
School Sport New Zealand said as of last night the 79 other sport tournaments that are due to host 20,000 students this week were going ahead, but it was in close contact with health officials.
Counties Manukau District Health Board has trained 25 nurses to carry out vaccinations at schools.