ACT Health said two people lived in the territory and the third was a person who spent time at Canberra Airport.

The ACT has averaged about three cases a year over the past five years.

The warning comes as New South Wales faces what could potentially be its worst measles outbreak in five years and a similar spike in cases nationally.

According to the federal Department of Health, even though Canberra has a comparatively high proportion of two year olds immunised, it was still below the levels needed for her immunity.

The ACT's Measles, Mumps and Rubella vaccine immunisation rate for two year olds is 94.5 per cent last year, compared with the national rate of 93.47 per cent. A 95 per cent rate is required to achieve herd immunity.

Acting ACT Chief Health Officer Dr Kerryn Coleman said despite the territory not facing the challenges of other states, vigilance was still important.

"Here in the ACT, we are fortunate we have not seen the same increase in measles cases that some of the larger jurisdictions have recently, however, this does not mean we won't," she said.

Dr Coleman said acquiring immunity to measles was simple.

"Every case of measles that is identified is a strong reminder that the best way people can protect themselves and their families against measles is vaccination," she said.

"Anyone who has not had two doses of the vaccination is at the greatest risk of contracting measles if they come into contact with someone who is infectious."

Extra care should be taken by anyone travelling overseas or interstate, especially as school holidays draw closer.

"Measles continues to circulate in many overseas countries and in other states and territories, in particular NSW. Therefore it is possible that ACT residents travelling outside of the ACT could be exposed to the disease," she said.

"People travelling should check their immune status before leaving home."

ACT Health was working with GPs, schools, daycare centres and residential aged care facilities to help control the spread of measles and other communicable diseases.

The vaccine is free from GPs to those born during or after 1966 who aren't yet fully vaccinated.

More information on immunisation is available from ACT Health.