SYDNEY - An 18-year-old man was arrested in Sydney on Tuesday over an alleged "imminent" terror plot, as police raided properties in Melbourne in a separate counter-terrorism operation, officials said.
Australian Federal Police claimed the teenager was scouting possible sites in Sydney to undertake an attack while at the same time trying to acquire a firearm.
In February he had been stopped at Sydney airport as he attempted to leave for Syria, despite his passport being cancelled, to allegedly fight overseas with jihadist groups.
"Since that time, he has been under the surveillance of the relevant authorities who have decided this morning to act to prevent an imminent terrorist event," Attorney-General George Brandis told reporters, without giving precise details of potential targets.
The teen, from Sydney, was expected to be charged with acting in preparation for, or planning, a terrorist act, which has a maximum penalty of life in prison.
He will also likely be charged with "preparing for an act of foreign incursion".
"This is the ninth occasion since the national terrorism alert level was raised (to high) in September 2014 that police have successfully intervened to prevent an imminent terrorism event on Australian soil," Brandis added.
Counter-terror police have made a string of arrests since late 2014, including a 16-year-old boy charged with preparing an attack linked to Anzac Day services honouring Australian soldiers in Sydney in April.
Other arrests saw a 17-year-old boy picked up in a raid in Melbourne a year ago, allegedly with "improvised explosive devices" in his family home.
Officials said Tuesday's arrest had no connection to police raids carried out in the southern city of Melbourne, part of an operation connected to five men detained last week over an apparent plan to leave the country by boat.
Police have alleged that the five -- aged in their twenties and thirties -- wanted to travel by boat to Indonesia and from there make their way to Syria to join jihadist groups.
The government has passed numerous national security laws including legislation allowing passports to be cancelled to prevent Australians from leaving the country.
But police have been unable to prevent all attacks, including the terror-linked murder of police employee Curtis Cheng by a teenager last October.