David Mearns, who has recovered more than 50 ships - including the British battleship HMS Hood, sunk by the German battleship Bismarck in World War II - said it seemed highly improbable the wreck off Dirk Hartog Island was the Sydney.
"I would love for this to be the Sydney but in my professional opinion it's not believable," he said.
Mr Mearns said German logbooks showed the November 19, 1941, battle between the German raider SS Kormoran and the Sydney took place 120 nautical miles west of the Gascoyne Coast, or 160 nautical miles southwest of the town of Carnarvon.
The wreck detected last week by amateur researcher Phil Shepherd and his mates - including master diver Ian Stiles, who filmed parts of the wreck with a grappling hook and an underwater camera - was just 20 nautical miles from Dirk Hartog and close to the coast.
The men would not elaborate on their find yesterday because of a confidentiality agreement with a media organisation, Mr Stiles's wife, Sharon, said.
A spokesman for Defence Minister Brendan Nelson said the navy would examine the wreck to determine if it was the Sydney.
Even the Fairfax media has had to back off its euphoric, and what now appears to be spurious, claim at the weekend that the sighting was almost certainly the Sydney:
The navy will work with the Western Australian Maritime Museum to try to verify claims that HMAS Sydney has been located off the West Australian coast. The move came after the sighting of a wreck which could be the vessel that sank on November 19, 1941, with 645 aboard. An investigation was necessary, a Defence Force spokesman said, before any "positive identification of the wreck" could be confirmed. Divers claimed last week that the remains of the vessel, sunk by enemy fire, had been found near Dirk Hartog Island."This is wonderful news," Mike McCarthy, the curator of maritime archeology at the West Australian Museum, said.
"The navy will probably send a survey ship with sonar capability to investigate the wreck, which is justified because of the huge interest this discovery has created," Dr McCarthy, who is advising the navy, said.
A spokesman for Bruce Billson, the minister assisting the Minister for Defence, said the Government was waiting for a report from the team that claims to have found the ship before committing a ship to the search."They have a lawful obligation to file this report, and once the navy has verification [it] can commit a survey ship, which would probably be a minehunter with sonar capability," the spokesman said "Until then it would be irresponsible of them to prematurely raise the expectations of relatives who have been living with this mystery for more than 60 years and want closure."Tonight's 7.30 Report summed up the current state of play well in words, though some of the accompanying video footage was grossly inaccurate: many of the warships depicted bore little resemblance to the Sydney. Can't the ABC researchers do better than this? All they need to do is to consult some of the knowledgeable people who were interviewed on the program.