The director of communicable diseases for the NSW Health, Vicky Sheppeard, said the warning was issued because all four people had recently been in places where they could have infected others.
said it was not an outbreak yet, but there was a possibility that others had been infected.
"We need to get the message out, so that people can be aware of measles symptoms if they are susceptible," Dr Sheppeard
said infants under the age of one were susceptible to contracting and people aged in their 20s and 30s who had only received one dose, or no doses, of the measles vaccine might also be vulnerable to the infection.
"Until you've had two dozes of measles vaccine you are considered susceptible," she
"All four cases came through Sydney Airport.
"We've also had one of the cases at Royal North Shore emergency department and another case at Mt Druitt Hospital
emergency department and a medical centre at Blacktown."
said in NSW all health workers were fully vaccinated, so health workers were not thought to be at risk and the patients that had been identified had been treated.
"But there might be others who were perhaps walking through the emergency department and not able to be identified and contacted and are at risk, so it's important to get the message out to those people as well," she
Measles highly infectious, travels through the air
said measles was the most infectious disease known to man.
"In 2012, we had one case of measles that resulted a further 169 cases over the period of a few months," she