A Waikato person who contracted measles while on an overseas trip has passed the disease on to at least two others, prompting health authorities to issue a warning that many more people are at risk.

One of the three who were affected attended Nga Taiatea Wharekura school while infectious but before becoming unwell. The school has been informed and is working with the Waikato District Health Board's Population Health to control the spread of the disease.

It is unknown whether the infectious person was a student or employed by the school.

During their infectious periods, the three infected people also attended Waikato Hospital's emergency department respectively on Friday, March 25; Monday, April 4; and Saturday, April 9.

Medical Officer of Health Felicity Dumble said anyone who attended the hospital's emergency department on those dates might have been exposed to measles.

People who shared the same air as someone infected with measles, even just being in the same room, is at risk of developing the disease if they are not immunised.

"These are the same measures we had to put in place during the outbreaks of 2014 and 2015. We learnt very early that we had to be very strict with the quarantine and, by being strict, it will prevent further cases at the school," Dumble said.

"It's a timely reminder to everyone else to check that they and their children are fully immunised against measles. Vaccination affords full immunity in the vast majority of cases."

Information has been circulated regarding the cases to the hospital emergency department and general practices in the region.

"Measles can be a very serious illness, with one in three sufferers experiencing complications such as ear infections, pneumonia, bronchitis, or diarrhoea. On average, one in 10 cases will require hospitalisation," Dumble said.

In 2014, there were 124 cases of measles in Hamilton associated with an outbreak.

Non-immunised people who have had contact with a person with measles will normally be advised to stay at home and away from all public places, school, or work for 14 days after contact.

"Immunisation protects not only the individual, but also blocks the spread of this disease within our communities."

Measles is a vaccine-preventable disease. The disease is spread by tiny droplets in the air and is one of the few diseases that can spread easily to those nearby.

Those who have had the Measles, Mumps and Rubella (MMR) vaccination are considered immune. People most at risk of contracting the disease are those who have either not had the Measles, Mumps and Rubella vaccine, or who have just had one dose of the vaccine.

If exposed, there are a number of measles symptoms to look out for.

The first symptoms of measles are a fever and one or more of a runny nose, cough and sore, red eyes. After a few days, a red blotchy rash comes on, usually starting on the face before spreading to the body. The infection lasts up to one week.

The incubation period for measles is approximately two weeks, meaning it can take up to two weeks from exposure to show symptoms.

Measles can be very serious. If you or your child becomes unwell, phone (don't visit) your GP or call the Health Line on 0800 611 116 for advice, or otherwise seek medical attention.

It is important to call first because measles is highly infectious and people with measles can infect others around them, such as in waiting rooms of GP surgeries or the emergency department.