Kiwis travelling to Hong Kong on an Air NZ flight earlier this month have been warned they were exposed to measles.

Tauranga resident Bryony Cross was "among more than 300 other passengers" on the April 1 flight as she and her husband flew to the Asian mega-city to watch the Hong Kong Sevens rugby tournament.

But last Thursday, having returned home, she received an email from Hong Kong's Centre for Health Protection stating there had been a confirmed case of measles from the flight.

The centre's email was being sent out to all passengers to warn them to look out for symptoms of measles - including fever, rash, cough or runny nose - developing between April 8 and 22.

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Cross told the Herald she and her husband had measles as children - fortunately with "no ill effects" - and their children had been vaccinated against it.

She said it "beggars belief" that some people chose to not vaccinate their kids.

"Hong Kong Stadium holds 40,000 people and the event was held over three days with a huge cross section of ages attending and all in close proximity," she said.

"If anyone had been infectious this would have spread like wildfire among those who had not been vaccinated."

It comes as measles outbreaks have been reported in locations across the globe.

In New Zealand there were 18 confirmed cases of measles in Auckland as of Friday, while five confirmed cases had been recorded in the Bay of Plenty over the past month and more cases reported in Christchurch.

In the US, a New York county said it planned to ban unvaccinated children from all public spaces after more than 150 cases had been confirmed as of late last week.

The county of Rockland declared a countywide state of emergency that will last for 30 days or until all unvaccinated children receive the measles, mumps and rubella vaccine.

An outbreak has also been recorded in Hong Kong, with Cross saying it was being reported in local papers over there and that even staff at the city's airport were contracting the illness.

Measles is a highly infectious virus that can be very serious.

It spreads easily from person to person through the air, via breathing, coughing and sneezing.

Anyone unimmunised who has been in the same room as someone with measles will likely get it, the Canterbury District Health Board says.

Dr Jim Miller from Toi Te Ora Public Health in the Bay of Plenty said GPs and residents were now on alert after the confirmed cases in the area.

"Immunisation is the best protection from measles and the vaccine that protects against measles is the MMR (measles, mumps and rubella) vaccine," he said

"It's important that parents ensure that their children receive their free routine MMR immunisations on time at 15 months and 4 years of age."

Dr Miller said for anyone who had never had a dose of the MMR vaccine now was the best time to get one and it was highly effective.

"After one dose of MMR vaccine, about 95 per cent of people are protected from measles, and 99 per cent of people who have had both doses are protected from measles," he said

The Ministry of Health recently highlighted that since 2012, all cases of measles in New Zealand came from travellers bringing the disease from overseas and that there are significant measles outbreaks in many countries.

People born before January 1, 1969 are likely to be immune because measles used to be quite common and so this older age group does not need measles immunisation.