The teenager from Georges Hall and a 20-year-old man from Raby were arrested in raids on properties in the city's southwest today and charged with conspiracy to conduct an act in preparation of a terrorist attack.
Ibrahim Ghazzawy (20) is expected to face Campbelltown Local Court this afternoon.
Three more men, who are already on remand facing related and unrelated matters, are also expected to be charged over an alleged plot in December 2014 to target government buildings, the Australian Federal Police say.
AFP Deputy Commissioner Michael Phelan says today's charges stem from investigations after material was seized under Operation Appleby in Sydney during September and December 2014.
Police allege the conspirators were involved in "formulating documents connected with preparations to facilitate, assist or engage a person to undertake a terrorist act".
The plot "clearly talked about a plan and there were government buildings named in those plans, specifically the AFP building," Mr Phelan told reporters at AFP headquarters in Sydney.
"The planning documents were evolving so specifically there's one mention of AFP. The details in those documents will come out eventually in court."
He would not say how far along in planning the attacks the men allegedly had gone.
"How well those plans were developed is a matter that will go before the court," Mr Phelan said.
Joint Counter Terrorism Team (JCTT) officers have been keeping the alleged conspirators under watch since plots were disrupted last December, he said in response to questions about whether the arrested men had continued plotting an attack.
"There has been an ongoing conspiracy we're alleging," he told reporters.
The teenage boy was 14 years old when the alleged terror plot was being formulated in 2014.
"It's disturbing that we're continuing to see teenage children in this environment," NSW Police Deputy Commissioner Catherine Burn said.
"There's absolutely no doubt about the fact that we're charging a 15-year-old with a very, very serious offence - this is an offence that has a maximum of life imprisonment," she said.
Mr Phelan said the parents of the 15-year-old boy were "very cooperative" as their son was arrested this morning.
But Ms Burn said it was a concern that the teen had become so radicalised and that police had been working with the boy's school.
"We don't know how the 15-year-old has got to the point where we will allege he got," she said.
"The principal and the executive of the Department of Education are aware of today's activities."
Today's arrests come a little more than two months after a teenage gunman, 15-year-old Farhad Jabar, shot down police accountant Curtis Cheng outside police headquarters in Parramatta.
"I am not aware of a direct connection between the two 15-year-olds. However, the people who were involved with the murder of Curtis Cheng are associates of these people," Ms Burn said.
She said today's arrests were not linked to last week's raids when officers swooped on two homes in Merrylands, including the Lockwood Street home of a man accused of supplying the gun that killed Mr Cheng on October 2.
Those raids followed reported Facebook threats made against the Merrylands police station.
Talal Alameddine (22) remains in police custody charged with supplying the gun to Jabar.
