Fifty years after Paul Gilmore threw a message in a bottle into the Indian Ocean, he's finally had a reply. But he doesn't know it yet, because he's back at sea.

Key points: Mr Gilmore's letter was found by a nine-year-old on a remote South Australian beach

Mr Gilmore's letter was found by a nine-year-old on a remote South Australian beach It sought a penpal for the then 13-year-old in 1969 who was moving to Melbourne

It sought a penpal for the then 13-year-old in 1969 who was moving to Melbourne The ABC managed to track down the author through family in Australia and the UK

"It's amazing, absolutely incredible," his sister Annie Crossland laughed. "He'll be chuffed to bits."

On November 17, 1969, the then-13-year-old was travelling from his home in England to the other side of the world and set loose a letter seeking a new penpal.

On Tuesday this week, that penpal finally wrote back. Nine-year-old Jyah Elliott, who found the bottle on South Australia's Talia Beach on the Eyre Peninsula, dropped his reply in the post the very same day.

Jyah Elliott was fishing with his Dad Paul when he found the bottle. ( Supplied: Carla Elliott )

"He was so excited," his mother, Carla Elliott, said, although Jyah initially thought the letter was a fake.

The ABC tracked down Mr Gilmore through his extended family in Australia and the UK.

But he himself couldn't be reached, because he's out at sea again — this time in the Baltic.

"He's on a cruise now," Ms Crossland said.

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

Nearly 50 years ago, the Gilmores were '10 pound Poms' heading to Australia aboard the TSS Fairstar, a 21,619-tonne steamship.

The Gilmores lived in Australia until 1973 and then moved back to England.

Ms Crossland, who was on board the steamer with her older brother, remembered him "writing letters and putting them in bottles".

"He sent about six of them," she laughed.

"So it's good that one of them has surfaced.

"I don't remember where he got the bottles from … [but] I remember my dad saying it cost him a fortune in drinks on the ship."

"So I don't know what sort of bottle it would've been — it would've been a bottle of Coca-Cola or something."

Jyah found the letter on Talia Beach, which sits on South Australia's Eyre Peninsula. ( Supplied: Nicole Ely )

An ocean voyage from UK to Australia

The Fairstar left from Southampton in November that year — sailing via the Canary Islands and Cape Town — before landing in Fremantle a month later.

After a short layover, the Gilmores carried on their journey to Melbourne.

It's believed the message was dropped somewhere in the ocean during this stretch of the journey.

Mr Gilmore threw the letter overboard when he was over 1,000 kilometres east of Fremantle, WA. ( Supplied: Carla Elliott )

"It was exciting and it was a big adventure," said Ms Crossland, who now lives in the north of England.

"It was the first time we'd been abroad anywhere, and we were looking forward to having a new life in Australia."

Mr Gilmore's younger brother, David Gilmore, said their father decided to take the family to Australia in the hope it would lead to a better life.

"I think my dad had had too many winters working outside, building houses, and was told that Australia was the land of opportunity," he said.

David was only four at the time, but said he recognised his brother's handwriting in the message.

"It's really strange," he told the ABC. "I'm looking at the message now and yeah, I can see it's my brother's writing — he's obviously a bit younger then.

"I'm amazed. It obviously brings back fantastic memories of moving to Australia."

In the letter, the teenage Paul Gilmore inscribed his future address in Mitcham, Victoria, and wrote: "We are 1000 miles east of Fremantle, Western Australia … please reply."

Jyah sent his reply to the Mitcham address. He's very unlikely to get a response, but it was just as unlikely then — David Gilmore said the family moved to another address in Ringwood just months after arriving.

But Paul's siblings have assured the ABC that when Paul finally returns from his latest journey, Jyah should expect another letter.

It shouldn't take quite so long this time around.