The family of a missing Irishman in Australia have launched an Internet campaign to track their son down.

Sean Walsh, 26, from Waterford, was last seen in the Sydney suburb of Randwick on Sunday, May 10, in the early hours of the morning, Australian police say.

He was due to fly back to Ireland on Tuesday having been in Australia since November last.

Sydney police say they have “serious concerns” for Sean's welfare.

His sister, Brid Walsh, told an Australian paper that it was completely out of character for him to go missing, adding that he had been in regular contact with home. She said that he had a very close relationship with his family.

A Web site, has been set up, and alerts have been sent out on various other social networking sites, including Facebook and Twitter.

Gill Mahony, a friend of the Walsh family, told IrishCentral that she had sent out an email to around 10 friends looking for information on Sean a few days ago, and from there it had gone around to thousands of people, eventually ending up on a Lonely Planet guide book Internet forum.

In the email, Mahony said that Sean was an “extremely good natured and a fun loving person.”

She said that his two sisters, Brid and her Mairead are currently in Sydney searching for their brother, and told IrishCentral that they had appeared on Australian radio appealing for information.

“They are only two people and any information would be greatly appreciated,” Mahony said in her email. “We all know someone over there and you would never know who could have seen him in Sydney over the weekend.”

Mahony told IrishCentral that Sean had previously visited Japan and New Zealand, and was a qualified engineer.

Mahony said that it was a very difficult time for Sean's family, and said that they were extremely anxious for his safety.

Australian police say that since he was reported missing, they have searched for him using police helicopters, without success. 

Sean is described as being 175cm tall (5 ft 10) with a thin build, shaved brown hair, blue eyes and a fair complexion. He was last seen wearing khaki board shorts, and a khaki-hooded sweater.