An "unrelenting mumps outbreak" hitting Auckland has health officials appealing to parents to immunise their children.
More than 130 cases have been reported in the city this year, compared with 35 in 2016, the Auckland Regional Public Health Service said.
In 80 percent of cases, the patient has not been fully vaccinated.
"It is disappointing because mumps is a preventable and serious disease," the service's clinical director Julia Peters said.
"Most recover from this disease. However, in the last six months a number of people have suffered from severe complications."
This included some adolescent males who were treated in hospital for pain and swelling in their testicles, which in rare cases can result in infertility.
In other cases, some females experienced ovarian inflammation and another person developed meningitis.
Non-immune pregnant women who catch the disease risk miscarriage in the first three months.
In rare cases, mumps can cause permanent hearing loss.
With almost 70 percent of this year's cases occurring in those aged between 10 and 19, Dr Peters urged parents to ensure their families' free measles, mumps, and rubella (MMR) vaccinations were up to date.
Mumps can spread quickly among those who are not immune, and one infected individual can cause an outbreak at a secondary school, because immunity in that age group is well below the national average.
"We are in the midst of an outbreak and already many students are scrambling to catch up on school work after missing school for several weeks," Dr Peters said.