An Australian boy who discovered a message in a bottle on a remote beach may have a new penpal after the Englishman who dropped it from an Australia-bound ship 50 years ago was tracked down.

But Jyah Elliott, aged nine, may have to wait some time as the message's now 63-year-old author is once again at sea -- this time on a cruise in the Baltic, the ABC reported.

Jyah discovered the decades-old treasure in sand dunes on a South Australian beach on Tuesday and by the next day the writer had been located.

Paul Gilmore was just 13 when on November 17, 1969 he dropped the bottle into the Indian Ocean hoping to reach a new friend.

The letter, which gave the ship's location as "1,000 miles east of Fremantle, Western Australia, asked its recipient to "Please Reply".

Although Jyah initially thought the letter was a fake, he was quick to post a reply on Wednesday.

Read also: Oldest message in a bottle found at remote Australian beach

"He was so excited," his mother Carla Elliott told the ABC.

Gilmore's sister Annie Crossland told the ABC that her brother was currently cruising but would be sure to contact Jyah when he returns home.

"It's amazing, absolutely incredible," Crossland said. "He'll be chuffed to bits."

She said the family had long ago moved on from the reply address on the letter.

Gilmore sent six bottles from the TSS Fairstar, a ship that transported so-called "10-pound Pom" migrants from Great Britain to Australia.

"The last time he was on a ship was probably going to Australia," she said. "Cruises aren't his thing."