Passengers on an international flight to Sydney Airport and a train to Sydney's south-west have been warned to look out for measles symptoms, after a man was diagnosed with the highly-contagious condition.

The man in his 40s was returning from Bangladesh on Cathay Pacific flight CX139 on Tuesday May 28. The plane landed at Sydney T1 International Terminal at 7.40pm.

The man travelled on a Cathay Pacific flight to Sydney form Bangladesh.

He travelled by train from the airport on the Macarthur line, departing at 9.11pm and arriving at Leumeah at 9.58pm.

People who travelled on the same plane and train, and those in the international terminal - (including the baggage carousel, customs, and arrivals areas) between 7:40pm and 9:30pm should be alert for signs and symptoms of measles until June 20, NSW Health said on Thursday.