New Zealand is battling its worst measles outbreak this century, with health professionals battling to contain the disease among non-immunised Kiwis.

At least 844 people have contracted measles this year, with one researcher declaring the crisis "out of control" and creating chaos in hospitals.

With the number of cases still rising, University of Auckland vaccinologist Dr Helen Petousis-Harris said unless numbers began to trend down, a first Kiwi death since 1991 from the disease would occur.

"(If) you keep accumulating enough cases, it's ultimately inevitable," she told broadcasters Three.

Growing fears of deaths or long-lasting health problems among the infected, one of the hardest-hit cohorts is those aged four and under.

The other is young people aged between 15 and 24.

The infection rates are uncomfortably high compared with Australia.

Australia - around five times bigger than New Zealand - reported just 103 cases last year, though that number has jumped to 154 so far in 2019.

On Wednesday, 31 new cases were reported in Auckland alone.

New Zealand's biggest city was responsible for 731 of the cases as of Wednesday, with more than 500 in Counties Manukau; south and east Auckland.

Those suburbs hold a higher than average Maori and Pasifika population, and lower than average household incomes.

One hospital in Middlemore, has set up a dedicated measles wing.

In one effort to quarantine infections, schools have asked non-vaccinated children to temporarily stay away.

Health professionals don't believe "anti-vaxxers" to be chief cause of the outbreak, but instead low vaccination rates and a limited capacity to quickly respond.

The government has announced new immunisation services and nurses to fight the outbreak, suggesting they could be stationed at malls, schools or churches to improve coverage.

Public health officials have also asked anyone travelling to Auckland to be vaccinated at least two weeks in advance of visiting the city.

They've also lowered the suggested age that babies be vaccinated, from 15 months to 12 months of age.

This year's cases have already doubled the size of the two latest outbreaks, in 2011 and 2014.

Cases were not contained to Auckland, with fresh reports in Taranaki and South Island resort town Queenstown also recorded this week.

The "MMR" vaccine - for measles, mumps and rubella - is not compulsory in New Zealand, but is free from a general practitioner.