One of Australia's oldest naval mysteries has been solved after the discovery of the wreck of the country's first submarine – more than 103 years after its disappearance in World War I.

Among its 35 crew members was a New Zealander, Able Seaman John Reardon. The NZ Navy Museum says he was the first Kiwi to die on active operations in the war.

The AE1 vanished off the New Guinea island of New Britain on September 14, 1914 - as well as Reardon, crew members hailed from Australia and Britain, the Daily Mail reported.

It was the first Allied submarine loss of the war and the first wartime loss for the Royal Australian Navy, yet the exact reason for its sinking remains unclear.

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Fish swim around the helm of the Australian submarine HMAS AE1 off the coast of the Papua New Guinea island of New Britain. Photo / AP
Fish swim around the helm of the Australian submarine HMAS AE1 off the coast of the Papua New Guinea island of New Britain. Photo / AP

No fewer than 12 fruitless hunts for the sub had been carried out over the past several decades, but Australian Defence Minister Marise Payne said on Thursday that it had been located more than 300m (984 feet) below the surface in a search using a Dutch-owned survey vessel that started only last week.

While the reasons for the submarine's sinking remain unclear, Payne said the Australian government was now trying to contact descendants of those killed on board.

"It was the first loss for the RAN and the first Allied submarine loss in World War I - a significant tragedy felt by our nation and our allies," Payne said in a statement.

Payne said a commemorative service was held to remember those who died after the vessel was found. Australia will now discuss with the Papua New Guinea government the building of a lasting memorial and ways to preserve the site.

The AE1 made final contact with an Australian ship at 2.30pm the day it disappeared. Mystified villagers on a nearby island at the time spoke of seeing a "monster" or "devil fish" that appeared and quickly disappeared into the water.

It was the first Allied submarine loss of the war. Photo / AP
It was the first Allied submarine loss of the war. Photo / AP

It has always been assumed the AE1 was not a victim of enemy action, since the only German vessel nearby at the time was a small survey ship.

Because no wreckage, oil or bodies were found, it was also believed the AE1 sank intact, most likely after striking a reef that punched a hole in the pressure hull. Whether or not this is what happened is still to be publicly verified.

The Australian navy submarine AE1 sank off the island of New Britain in September 1914. Photo / supplied
The Australian navy submarine AE1 sank off the island of New Britain in September 1914. Photo / supplied

In 2014, the NZ Herald's Andrew Stone reported on the disappearance of John Reardon and AE1.

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Stone wrote of Reardon: His name is not etched on any memorial in this country but can be found in Canberra on the Australian War Memorial. A dark-haired man with fresh complexion, Reardon, who came from Kaikoura, left New Zealand to go to sea. In March 1913, at the age of 22, he signed up to the Royal Australian Navy. Service records note Reardon, who signed up for five years, had an anchor and clasped hands tattooed on his left forearm.

Unlike New Zealand, which opted for its own unit of the Royal Navy, Australia formed a navy to defend its territorial seas. As part of its fleet it commissioned two submarines - novel and highly-secret vessels at the time powered by two diesel engines and armed with four torpedo tubes.

Able Seaman John Reardon of Kaikoura became the first New Zealander to die on active service in World War I, with the sinking of the Australian navy submarine AE1 in September 1914.
Able Seaman John Reardon of Kaikoura became the first New Zealander to die on active service in World War I, with the sinking of the Australian navy submarine AE1 in September 1914.

Reardon was assigned to the crew of AE1, possibly on transfer from the Royal Navy. Crew selection followed tough training to prepare sailors for underwater duties.

The pioneer vessels, 55m long and weighing 660 tonnes, set off on their 21,000km maiden voyage from Portsmouth to Sydney in March 1914. It was an eventful trip: a prop fell off one sub, and mechanical failures forced them to be towed for nearly half the distance. Through the Suez Canal, to beat soaring temperatures inside the crammed 6.7m wide craft, AE1 was painted white.

On May 25, Empire Day, the subs reached Sydney - the end to a "most wonderful journey of endurance, both for men and engines," wrote engine room hand John Marsland in a diary of the trip.

Repaired and refitted, the subs were tasked to attack the German Pacific Fleet when war was declared and went hunting for enemy ships near Rabaul in what was then German New Guinea. AE1 - which one tribe called "a devil fish" - was part of an Australian flotilla when, at 3.30pm on September 14, contact was lost in poorly charted waters full of dangerous reefs. It was the last ever seen of AE1 and her crew, including Able Seaman Reardon.

Its final resting place has eluded searchers ever since, including the famed undersea explorer Jacques Cousteau. Reardon is remembered on a plaque at the Garden Island Naval Heritage Centre in Sydney which honours the 35 men lost when AE1 vanished. "Entombed but not forgotten," it reads.

Reardon was from a large Kaikoura family who were involved in whaling.

A great-niece, Marilyn Abernethy, 79, who lives in the South Island east coast town, said of the finding of the submarine: "It's really exciting. It puts a closure to everything."

Abernethy wants AE1 left where it is. She hopes a memorial bearing the names of the dead can be built on the closest island.

A memorial stone for Reardon which was placed in the Kaikoura cemetery by his mother fell during the earthquake last year.

Navy historian Gerry Wright, who has studied the loss of the AE1, said: "From the photo, the sub clearly nose-dived to the bottom so the end was quick."

"The lack of urgency given to AE1's motor fault shows that she was not deployed in German New Guinea as a submarine but as an extra mobile platform to watch for any approaching German naval force."

Consequently the sub wasn't prepared for diving. The canvas shelter around the bridge and radio mast were not unrigged as they would need to have been for diving.

Wright suggests the sinking may have been linked to problems with a torpedo tube and the fact that E class submarines had poor buoyancy at the bow (front).

"The 18-inch [457mm] torpedoes of the day needed constant maintenance. To maintain a tube-loaded torpedo it first had to be withdrawn from the torpedo tube.

"Had a torpedo tube's safety interlock failed for any reason, especially in the bow tube, while the outer door was also open, the rush of water would have pushed the torpedo back into the submarine and forced all the nearby men away from the torpedo tube controls.

"The submarine, then increasingly heavy as it flooded, would have quickly nose-dived to the bottom."

NZ Defence Minister Ron Mark said the loss of the AE1 was a "tragedy of war, but it's great that the submarine has been found after so long, and the crew's descendants, including those of our own Able Seaman John Reardon, have some closure at last.

"Senator Payne called me last night with the news, and we'll be talking again about it soon.

"I certainly hope that in time there will be a suitable memorial which people can visit, and that the site will be treated with the respect those brave sailors deserve."