A Sydney teenager accused of planning an "imminent" terrorist attack was in the process of getting a firearm when he was arrested, Attorney-General George Brandis says.
The 18-year-old was arrested in Macquarie Park, in Sydney's northwest, on Tuesday in a joint operation with the AFP and NSW police.
The teenager was allegedly scouting for possible attack sites in Sydney that involved "buildings representing authority".
Police also revealed he tried to board an international flight from Sydney Airport in February but was stopped because his passport had been cancelled.
"Since that time, he has been under the surveillance of the relevant authorities who have decided this morning to act to prevent an imminent terrorism event in Australia," Senator Brandis said in Brisbane.
It was the ninth time since the terrorism alert level was heightened in September 2014 that authorities had thwarted such a plot, he said.
The man was "in the process" of acquiring a weapon.
"The judgment of the police was that he was intending to put it to use imminently," Senator Brandis said.
Counter-terrorism officers left it until "relatively late" before they acted so more evidence was gathered.
"Of course, a fine judgment has to be made as to how late one waits," Senator Brandis said.
"The police never wait so long that public safety is put at risk."
AFP Acting Deputy Commissioner Neil Gaughan said police were satisfied any threat to the community posed by the teen was mitigated.
"This young person's life changed today," he said.
"When he made those decisions he took two weeks ago, he's now looking at life in prison."
Senator Brandis said although the teen was acting alone, police believed he had connections to other individuals who were known to authorities.
The teenager was charged with one count of planning a terrorism attack and one of preparing for foreign incursions.
He will face Parramatta Local Court on Tuesday afternoon.
