And now he has a name: Thomas Welsby Clark.

It was a gruelling two-decade long process to identify the 21-year-old Able Seaman whose body washed up in a lifeboat on Christmas Island months after the warship was attacked in November 1941.

Able Seaman Clark was believed to have been the only person to survive and was found with tucked legs, meaning that despite having shrapnel in his head and floating alone, he still tried to row himself home.

After speaking to the Able Seaman’s niece, Veterans’ Affairs Minister Andrew Gee said knowing he was at sea alone for all those months caused anguish for the family.

But burying the body under a marked headstone also brought a sense of pride.

“By identifying Tom, our nation honours all those who lost their lives in HMAS Sydney,” Gee said.

“His story helps Australia understand the immense sacrifice made for our country and also the loss and grief that is still felt by the descendants of those who perished on that day.”

The Brisbane born Clark signed up to the forerunner of the reserves in 1939 before deciding to join the Navy in August 1940.

He gave the ultimate sacrifice 15 months later.

While the Able Seaman’s identification has given hope to the families of soldiers who never came home, it was also met with mixed feelings from some of the other 644 families who lost relatives.

Some of the 170 relatives of sailors whose DNA was tested to confirm Able Seaman Clark’s identity were initially reluctant to participate for fear it would confirm it wasn’t their brother, son, uncle or cousin.

“Water bares no battle scars and leaves no trace to history,” Vice Admiral Michael Noonan said.

“After 80 years, we return him the dignity of his name.”

The sinking of the HMAS Sydney at the hands of the disguised German merchant raider HSK Kormoran was a terrible day in Australia’s history after Europe had descended into darkness and the United States was yet to join the war.

Gee lamented the sacrifice made by the people aboard the Sydney, who fought until their last breath.

“The loss was devastating, not only for families but it was a great hit to Australia’s morale during the Second World War – it shook our nation,” he said.

“It was only a few democracies like Australia who stood up and fought against tyranny, and that is what the crew of Sydney were doing.”

More than 360 sea mines were going to be laid by the German vessel off the coast of Australia.

It had already sunk 10 merchant vessels and captured one more.

“The Kormoran has been inflicting devastating losses on the high seas up until the day it met the Sydney,” Gee said.

“By removing the threat of the Kormoran the crew of the Sydney undoubtedly saved many, many Australian lives.”

Developments in technology mean more identifications are expected in the future.

At least 200 people were involved in the identification process and the vast majority did so on their own time and dollar to help return the seaman to his family.

The bodies of more than one third of all soldiers in WWI have no known grave.

This morbid statistic reduced in subsequent wars as Australia “sadly got better at bringing bodies home”.

But in once sense, the countless others like Able Seaman Clark who lay in unmarked graves have “always been known to us as (their) name is on the honour roll upstairs”, Australian War Memorial director Matt Anderson said.