Health officials fear there could be an outbreak of mumps after 20 cases were reported in just five months - up to triple the usual rate.
Thirteen cases were reported in Auckland, while Rotorua and Taupo each had three cases. Cases were also reported in Wellington (two), Canterbury (one) and the West Coast (one).
The Ministry of Health said it appeared most of the cases had been acquired overseas, and had affected people between the ages of two and 31.
There was an increased risk of further outbreaks so people should check they were immunised, the ministry said.
Deputy director of public health Harriett Carr said the rise in mumps cases usually stemmed from people not being immunised.
"With these 20 cases among children and young adults, we think most have come from Fiji where there is a mumps outbreak at the moment," she said.
"There's always a chance people will pick up an illness and bring it back here when they're travelling, so the best thing we can do is make sure we have good immunisation rates and that people are aware of the symptoms of these illnesses."
Symptoms of the viral illness include sore and swollen areas under the jaw, usually accompanied or preceded by fever, muscle soreness, poor appetite, fatigue.
Many people were unaware that mumps could be a serious illness, Dr Carr said.
"About one in 10 people get meningitis and there are other, rarer serious consequences."
The start of the school year heightened the possibility of further cases, she said.
"People should keep their children home if they think they might have mumps and contact their GP and Healthline."