A case of measles has been diagnosed in NSW, the first in more than two-and-a-half years.

NSW Health says a person in their 50s acquired the highly contagious disease while travelling in Asia last month.

The person developed symptoms after returning to Sydney and is now isolated in hospital.

They spent time in the following locations while infectious: St Andrew's Catholic Church Malabar for the 10.30am mass on September 4, the University of NSW on September 6, and Pacific Square in Maroubra on September 7.

NSW Health's Jeremy McAnulty said those locations did not pose an ongoing risk but urged people who were there on those days to be alert for symptoms until September 24.

Measles is spread by the droplets from when an infected person coughs and sneezes.

Symptoms include fever, sore eyes and a cough followed three or four days later by a red, blotchy rash spreading from the head and neck to the rest of the body.

"If you develop symptoms, please call ahead to your GP to ensure you do not wait in the waiting room with other patients," Dr McAnulty said.

People considered susceptible to measles are those who have not had two lifetime doses of measles vaccine, and those who have a confirmed history of measles infection or a weakened immune system.

Herd immunity provides protection to those unable to be vaccinated such as babies and people with weakened immune systems.