Passengers and crew on two domestic Air New Zealand flights were exposed to measles as a woman on board travelled while infectious, it has been confirmed.

The woman, who is aged in her 20s and is from Queenstown, is one of six people in the South Island who have been diagnosed with the highly contagious disease during the past week. Public Health staff are yet to trace the source of the outbreak.

The woman left Queenstown for Christchurch on Flight NZ 5642 at 8.20am on April 7 and returned on Flight NZ 5653, which departed Christhchurch at 3pm on April 9.

Canterbury DHB's public health unit said anyone who was on those flights should check whether they've been immunised against measles.

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"If not immunised against measles, they may become unwell in the next week and should phone their doctor for advice," a spokeswoman said.

"They should not go to their general practice or turn up at an Emergency Department if they are unwell as they will risk infecting more people.

"Measles is highly contagious: If one person has it 90 per cent of the people close to that person who are not immune will also become infected."

Cases of the illness have been confirmed in Queenstown, Wanaka, Canterbury and Nelson-Marlborough.

Investigations are into the source of the outbreak are continuing, however the person who sparked it may have had a relatively mild illness and fully recovered.

Public Health Units at Nelson Marlborough, Southern and Canterbury DHBs were identifying the close contacts of infected people and working with affected individuals and work places to provide advice to staff.

Instead of going to the doctors people who believe they may have measles should phone their GP after hours or call Healthline on 0800 611 116.

About measles

• Measles is a notifiable disease, which means any confirmed cases must be reported to government agencies.

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• It is highly contagious and is spread through coughing and sneezing.

• Early symptoms include a dry cough, runny nose, temperature of more than 38.5 C and feeling very unwell.

• After four or five days a rash appears, it usually starts on the sufferer's face, then moves down to the chest and arms.

• Those confirmed to have the disease are infectious from five days before the onset of the rash to five days after the rash starts.

• People should stay in isolation while they're infectious - this means staying home from school or work and not having contact with unimmunised people.

• People are considered immune if they have had two doses of MMR (measles, mumps, rubella) vaccine, have had a measles illness previously, or were born before 1969.

• More information about measles is available at www.health.govt.nz/your-health/conditions-and-treatments/diseases-and-illnesses/measles