An 11-year-old visitor from Auckland has been diagnosed with measles, and a 14-month-old from Australia is suspected of having the illness, after presenting to Bay of Islands Hospital over the weekend.

There had been limited "public space exposure," Medical Officer of Health Dr Virginia McLaughlin said, but one of the children had been at the Ngāwhā Springs hot pools between 7pm and 8pm on Easter Monday, and anyone who had been there at that time was encouraged to contact the Northland DHB's Public Health Unit (on 0800 600-720).

The unit had contacted, or was contacting more than 49 people who had potentially been exposed to the disease.

"It was only a matter of time before we saw measles in Northland, and given our low immunisation rate, and with many families travelling over the school holidays, the risk of being exposed to measles is far greater," she added.

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Measles was one of the most infectious diseases for humans, and under-immunised people who came within two metres of an infectious person, however briefly, had a 90 per cent chance of contracting it.

"People need to be very aware of the symptoms of measles, which starts with a fever, cough, runny nose and sore red eyes, then after three or four days a rash appears on the face and then spreads to the body," she said.

"You are contagious five days before to five days after rash onset, so in the early stages you may spread the infection without actually knowing you have measles.

"If you are found to be a contact of a case of measles (and not immune) you will need to stay away from work, school or public places for up to 14 days, to help prevent putting other people at risk." Exclusion periods would be enforced, the best way to avoid that, and to ensure the community was protected, being to bring immunisations up to date immediately.

"It has been estimated that to prevent recurrent outbreaks of measles, 95 per cent of the population must be immune. Presently Northland's rate is about 86 per cent, so we are well below target," he said.

"This is incredibly frustrating as a public health practitioner, practising in a developed country such as New Zealand. Our low coverage rate means that the most vulnerable people — babies and children too young to be immunised, and those with immunosuppressive disorders — are being placed at unnecessary risk. Make no mistake, measles can be a very serious illness, and is easily prevented by vaccination.

"Given our immunisation rates are very low it may be time to start looking at compulsory immunisation."

Only those born before 1969, or who had had two MMR vaccinations, were considered fully protected. Those aged between 29 and 50 would only have had one vaccination, and were not considered immune.

Modelling suggests those aged 14 to 37 years were most susceptible to measles. All New Zealanders are eligible for two free doses of MMR.

Dr McLaughlin advised anyone who had symptoms suggesting measles to immediately seek advice from a doctor, and avoid contact with young children. They should call ahead to alert their doctor about the possibility of measles to allow them to make arrangements to assess the patient safely without infecting others.

Emergency departments and after-hours clinics should be avoided. Outside GP hours, call Healthline on 0800 611-116 .