The landmark building is featured in the latest edition of "Inspire", an English-language site dealing with bomb-making and terrorism, reportedly run by associates of Al-Qaeda in the Arabian Peninsula, a militant Islamist group.
"Its aim is to literally inspire the lone wolf attack, in Australia and any other countries for that matter, it is circulated internationally," Attorney-General Robert McClelland told reporters.
"It is trying to equip usually disgruntled young men with the skills they need to develop weapons of destruction -- how to construct a bomb, how to use weapons... to actually undertake an act of violence."
McClelland said it was the "first time an Australian icon has been used in one of these publications" and it was "obviously a concern" but that he had been advised there was no direct threat and no need to raise the terror level.
Australia's terror alert level has stood at "medium" -- meaning an extremist attack could occur -- since 2001.
"It was presented as a photo image. There was no other content referencing it as a target or otherwise," McClelland said.
"The only way that (alert level) would be elevated is if our agencies received intelligence of a likely attack and that is not the case."
Though it underscored that "the threat of terrorism is real in Australia" McClelland said previous planned attacks had been detected and disrupted before they came to fruition, with 38 people charged and 23 convictions.
McClelland said the government had taken steps to have the site removed or blocked from display in Australia but acknowledged that removing all access would be difficult.
"In the modern age of global electronic communications, the reality is this material will emerge on overseas sites," he said.
New South Wales assistant police commissioner Peter Dein, the state's counter-terrorism commander, said authorities became aware of the Opera House picture on Tuesday.
"When you actually read it you see that most of it touches on the death of Osama bin Laden, but then you get to the back of the magazine and you find that there's a photograph of the Opera House," he said.
"I must stress that there's no text, no commentary that exists (in the magazine) that relates to the Opera House at all," he said, adding it was hard to interpret what message it might be sending.
Dein said he believed US-born Yemeni cleric Anwar al-Awlaki was behind the literature, describing him as one of the most dangerous men in the world.
"He is wanted by a number of countries for terrorism offences, he has influenced and played a significant role in a number of recent terrorist attacks," he said.