Train commuters and passengers who have travelled on Tiger Airways in Sydney are on alert after a measles-infected tourist spent four days exploring busy city attractions.

Authorities are warning people in New South Wales they were at risk of contracting the highly contagious disease potentially spread by the man who has returned home to Queensland.

NSW health authorities said the man, aged in his 40s, was infectious while travelling from the Gold Coast to Sydney on Tiger Airways flight TT609 on June 29.

It is believed he caught the measles, a potentially fatal disease that causes skin rash and fever, during a recent trip to Thailand.

After landing in Sydney, the man then took a train to Circular Quay, a busy tourist hot spot.

While in Circular Quay, he stopped at Cafe Rossini and took a ferry to Manly with a stop at Blue Water Cafe before returning back to the bustling port.

On Sunday, he spent the day with visitors in Sydney before travelling around the city again on Monday, when he took public transport from Berowra to Wynyard station.

From there, he spent the day at The Rocks Discovery Museum, The Australian Hotel and Starbuck’s at Hyde Park.

The man caught a train from Central Station to the airport on Tuesday morning and took Jetstar flight JQ420 back to the Gold Coast.

NSW Health has urged anyone who may have been in contact with the man to be alert for symptoms of the disease, which can take up to 18 days to appear.

People who have not been immunised are at high risk of measles infection. Photo: Centres for Disease Control

Measles symptoms include fever, sore eyes and a cough followed three or four days later by a red, blotchy rash.

More than 40 people have been diagnosed with measles in NSW since Christmas.

A recent report shows vaccination rates in NSW are at their highest levels, with more than 95 per cent of five-year-olds vaccinated.

-with AAP