A measles alert in New South Wales has been issued after a man was diagnosed with the disease upon returning from Southeast Asia, in the 40th New South Wales case of the disease since Christmas.

Measles symptoms include fever, sore eyes, a cough for three or four days followed by a red, spotty rash that spreads from the head down the body.

New South Wales Health communicable diseases director Vicky Sheppeard says none of the locations visited by the man pose an ongoing risk but it can take up to 18 days for symptoms to appear following exposure.