SYDNEY- Japan on Wednesday said the circumstances surrounding the sinking of an Australian hospital ship in WWII were unclear, after deep-sea searchers found the vessel's watery grave.

Australian Prime Minister Kevin Rudd has said the clearly marked hospital ship the Centaur was lost on May 14, 1943 at the cost of 268 lives when it was torpedoed without warning by a Japanese submarine.

The attack "struck deeply at the heart of our nation, and became a symbol of our determination to fight on against a brutal enemy," Rudd said last month as he announced the start of the search for the lost ship.

But a statement issued by the Japanese Embassy in Canberra said the circumstances in which the Centaur went down were not conclusive.

"The Japanese government had conducted its own inquiry into the Centaur," the statement released to AFP on Wednesday said, without giving any indication when the inquiry took place.

"The circumstances were not clear given that it occurred during the Second World War.

"We will see how the on-going investigation by Australia unfolds."

Australian searchers on Sunday said the underwater wreck of the AHS Centaur had been found off Australia's east coast and they would soon begin to film the remains using a remotely-operated submersible vehicle.

The search, which had focused on an area off Moreton Island off the Queensland city of Brisbane, found the wreck at a depth of over 2,000 meters (1.3 miles), they said.

In its statement, the embassy said since WWII Japan has "made the greatest efforts for world peace and prosperity as a responsible member of the international community and has also developed a close relationship with Australia."

"Japan will continue these efforts and work to maintain that relationship," it said.