A man who travelled on trains and buses across Sydney in the past week has been diagnosed with highly-contagious measles.

A measles alert has been issued for Sydney's inner west and other locations after a young man contracted the highly-contagious disease.

The man, aged in his 20s, visited Sydney's inner west, northwest and eastern suburbs while infectious in the past week, NSW Health said on Wednesday.

"(He) had not travelled immediately prior to his illness and has no known links to previously identified measles cases," the statement said.

The man travelled from Wollongong to St Peters via Central on December 27, before travelling by train from Sydenham to Bondi Junction and taking a bus to Bondi Beach.

He retraced the route about 5pm.

On Sunday the man caught a train from Sydenham to Berowra via Central about 11am before retracing the route about four hours later.

He also visited Marrickville Metro on Monday about 3pm before heading to a nearby GP on Tuesday.

Sydney public health expert Leena Gupta said the alert should serve as a reminder for everyone to check they are protected against measles.

The virus can survive in the air and on surfaces for a couple of hours.

"It is possible that other people have been exposed to this case so everyone needs to be on the lookout for the early signs of measles," Dr Gupta said in a statement.

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

All people born since 1966 need two measles vaccinations. Those unsure whether they've been vaccinated in the past can safely have another dose.