Waikato health officials are responding to a measles outbreak in the region, with six cases reported in two weeks.
Waikato District Health Board Medical Officer of Health Dr Richard Hoskins says the source of the outbreak is unclear. He is urging people to catch up on measles immunisations.
"The first two cases occurred in late December and we didn't find out about them until a week ago," Dr Hoskins told Newshub.
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"Those cases had plenty of opportunity for other people to have got sick or be getting sick."
He says anyone with symptoms - including fever, runny nose, coughing or red eyes - should call their doctor from home so they don't put others at risk.
Measles can also produce a red, blotchy skin rash. The disease is spread through the air from coughing and sneezing.
"The first symptoms of measles are a fever, and a runny nose or cough or red eyes. After a few days a red blotchy rash develops and lasts up to one week. The rash usually starts on the face and spreads to the rest of the body," Dr Hoskins said.
"We think it's likely there have been missed cases, either in the Waikato region or elsewhere, and that they may well have infected other people."
The best protection against measles is immunisation, health officials say. Two doses are needed of the measles, mumps and rubella vaccine to be fully immune.
The disease can lead to complications such as ear infections, pneumonia, seizures, diarrhoea and encephalitis (swelling of the brain).
The Ministry of Health says up to 30 percent of people with measles will develop complications - usually children under five and adults over the age of 20.
Schools and childcare centres have the legal power to exclude unvaccinated children when there is a risk of measles being spread.
If you or your child becomes unwell please phone your GP or call Healthline on 0800 611 116 for advice or seek medical attention depending on the severity of the illness.