A person, who potentially exposed hundreds of domestic passengers last week to measles, had ignored warnings to stay isolated.

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Passengers who flew onboard Jetstar Flight JQ237 from Auckland at 3.55pm last Thursday or Jetstar Flight JQ236 from Christchurch at 1.55pm on Saturday are being warned to check whether they're vaccinated.

"We were told of a case that had come down from the Auckland outbreak and obviously we have to act as quickly as we can," said the Canterbury medical officer of health, Alistair Humphrey.

The person had flown against advice from the Auckland Regional Public Health Service and put people at risk, Mr Humphrey said.

"Initially, it was a contact of a measles case, they had been asked to keep themselves isolated and they, thereafter, got on a flight and flew down to Christchurch against instructions," he said.

"They then developed symptoms so tracking back we were able to [find out] that, unfortunately, they were contagious while they were on the flight."

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Dr Humphrey said anyone who had contracted the illness onboard the flights would likely be able to transmit it others from about now.

"The people who are vulnerable are those on the flight who are under-vaccinated, that is those who have had either no vaccines or just one vaccination.

"It's impossible to know how many people on the flight would have been vulnerable in that way. Obviously if people are fully vaccinated they are not at risk."

Anyone who travelled on those flights is being urged to check whether they're fully immunised with their doctor. At-risk passengers should call their GP rather than show up to clinics to avoid infecting more people.

Passengers who aren't vaccinated are being told to stay in isolation until 22 June.

Measles facts: