Thirteen cases of measles have been confirmed in the South Island in one outbreak this month with four more suspected.
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The Ministry of Health is concerned the number of cases could rise as children return to school next week.
The disease is highly infectious.
The ministry's director of public health, Dr Caroline McElnay said parents should check whether their children have been properly vaccinated.
Adults are also at risk if they're not immunised, she said.
Seven of the current patients are 35 and older.
In the last major measles epidemic, in1991, there were up to 60,000 people infected and seven deaths.
