People exposed to a young child with measles could show symptoms by Monday.
The child, who is less than 15 months old - too young to have been immunised against the highly infectious disease - was on a Singapore Airlines flight from Ho Chi Minh City, Vietnam, that arrived in Auckland on 29 January.
The Wellington medical officer of health, Dr Annette Nesdale, says people could also have been exposed during a later flight that day to Wellington.
She says the child was at Wellington Hospital's emergency department in the early hours of 1 February and on the evening of 3 February, and was also seen at Karori Medical Centre on 2, 3 and 4 February.
People who may have been exposed should contact their regional public health service. Dr Nesdale says symptoms of measles include fever, tiredness, runny nose, a cough and sore red eyes.