Mathew Stewart had served as a private with the UN peace keeping force in East Timor before he was discharged from the Australian army on psychiatric grounds in 2001 and apparently disappeared overseas.
Counterterrorism authorities launched an investigation Wednesday when a man wearing a balaclava and speaking in what experts agree is an Australian accent appeared on Dubai-based Al-Arabiya television in what purported to be an Al Qaeda videotape made in Afghanistan.
Foreign Minister Alexander Downer confirmed the government suspected Stewart had joined Al Qaeda, although he had not been identified as the masked spokesman.
"We have reason to believe he is one of a number of Australians who have turned to Al Qaeda," Downer told reporters Friday.
"Any Australian who thinks that joining Al Qaeda is a way for the future is an Australian who is taking up arms against the Australian people and will find themselves, if captured, in enormous difficulty," Downer warned.
Stewart's family has confirmed police visited the family home on the Sunshine Coast in Queensland state on Wednesday and asked Vicki Stewart if she could identify her 28-year-old son from still pictures taken from the tape.
"Mrs Stewart had been shown photos and advised the federal police that it definitely was not Mathew," the family said in a statement issued Thursday.
"The family supports the work done by the federal police and is still grieving over Mathew who disappeared without a trace four years ago," the statement added.
Attorney General Philip Ruddock, who is minister responsible for the nation's top intelligence
"I can't tell you whether it is an Australian or not," Ruddock told reporters Friday. "Competent agencies are still undertaking a very thorough examination of the issue," he added.
Prime Minister John Howard said he would not confirm or deny that Stewart was a suspect.
A longtime friend of Mathew Stewart, Adam Miechel, told a newspaper he believed the terrorist spokesman who appeared in combat gear clutching an automatic rifle in the video was the keen surfer he grew up with.
"My first thought was, 'yeah, it even sounds like him,'" Miechel told The Daily Telegraph. "It looks like him. It sounds like him as well."
US forces reportedly found documents identifying Stewart as an Al Qaeda recruit when they raided a terrorist training camp in Afghanistan in 2002.
Australian intelligence agents had given priority to watching for Stewart after he crossed the Iranian border into Afghanistan on Aug. 4, 2001 -- a month before the al Qaeda attacks on the United States, The Australian newspaper reported. He had reportedly converted to Islam.
In the video, excerpts of which were shown on Australian television on Wednesday, the terrorist spokesman berated US President George W Bush and British Prime Minister Tony Blair over their involvement in Iraq.
He also claimed that a rocket attack on a helicopter that killed 16 US soldiers in Afghanistan in June was carried out by Al Qaeda fighters.
"The honorable sons of Islam will not just let you kill our families in Palestine, Afghanistan, Kashmir and the Balkans, Indonesia, the Caucasus and elsewhere," he said.
"It is time for us to be equals. As you kill us, you'll be killed. As you bomb us, you will be bombed," he added.