Health authorities are urging anyone with measles symptoms to phone but not visit their family doctor.
That comes as authorities warn that the latest measles outbreak now affects the entire South Island.
Two adults and a 13-month-old baby who were at Queenstown Airport on 22 March were first to get measles in the outbreak.
Canterbury Medical Officer of Health Ramon Pink said they were recovering well, but two new cases had now also been confirmed among men in their late 30s or early 40s in Nelson and Christchurch.
He said those people flew on flights around the South Island.
It's not known who the source was but Dr Pink said they had identified the strain involved.
"What we do know is that Queenstown is an international airport and we know that the strain of this particular measles virus has been also identified circulating in Australia.
"And so it's highly possible that someone who was unaware that they were infectious travelled to New Zealand, and because it is highly infectious, they could have infected people even just walking through and gathering for a period of time in an airport."
None of the first three was immunised with the MMR [measles, mumps and rubella] vaccine. Of the two latest cases, the one in Christchurch believed he was immunised but was unsure, and the Nelson man was partially immunised, with just one of the required two jabs.
Dr Pink said measles could kill and anyone with symptoms, which include a rash, should phone their doctor for advice rather than going to a GP surgery.
He said it was important that if there was concern that someone might have been exposed to measles that they call their GP or their local medical practice to let them know.
"The practice will then advise them of how they can manage them, particularly take some tests and then make sure that they exclude themselves at home until those tests reflect whether or not they have got measles."
Dr Pink added that those who are unwell but unsure whether they were immunised should call their family medical practice and check. As well, those unsure if they were vaccinated can get vaccinated for free, which would increase the protection levels in the community against measles.
Unimmunised people exposed to measles may get a dry cough, runny nose and temperature over 38.5°C and feel very unwell before a rash appears on day four or five of the illness, usually on the face and moving down to the chest and arms.
Dr Pink said technology now enabled the strain of the measles to be identified relatively quickly.
Public health authorities were aware, therefore, that the strain involved in the first three confirmed cases was also identified in late March - early April in Australia.
"I understand this particular strain may have been introduced into Australia by a traveller from southeast Asia. It's not a new strain, it's a strain that we have seen before but we certainly haven't seen it for some time."
Dr Pink also said that measles, which is a notifiable disease, can make people "really sick". It could also have complications, including ear and chest infections and neurological symptoms.
"And it also can cause death. It can kill people."
There were more than 21,000 cases of measles in Europe last year, including 35 deaths, according to information published by the World Health Organisation.
Good health services in New Zealand limited the spread of such communicable diseases, which made it easy for people to become complacent about the potential impacts, he said.
"It can impact on our health services and therefore it's a cost to everyone, physically, financially and socially."