Security footage shows radicalised teenager Farhad Mohammed giving an Islamic State-style one-fingered salute just minutes before he fatally shot police accountant Curtis Cheng, a Sydney court has heard.

Previously unseen CCTV footage shows Mohammed, 15, entering the Parramatta mosque about 4:13pm on October 2, 2015.

He knelt down to pray before leaving again wearing black Islamic dress and carrying a black backpack.

As he left the mosque, he looked directly at a security camera and held up his index finger.

Commonwealth prosecutor Paul Maguire SC told the committal hearing the gesture had "become known as the Islamic State salute".

The court also heard the teenager was found with a bloodied handwritten note in which he declared he would put "terror in the hearts" of non-believers.

"Your nights will turn into nightmares. Your days into hell," he wrote.

"Know that you are all being watched 24/7 while you are asleep, awake, planning.

Farhad Mohammad was found with this bloodied handwritten note. ( Supplied )

"By the will of Allah I have come here today to put terror in your hearts.

"And soon the mujahideen will do the same, by the will of Allah."

Mr Maguire told the court a handwriting expert had verified the writing as that of Mohammed, while a similar but not identical note found torn up in the teenager's wastepaper bin was found to be written by his sister Shadi.

Raban Alou, 19, Talal Alameddine, 24, Mustafa Dirani, 23, and Milad Atai, 21, are all accused of helping Mohammed carry out the fatal attack.

A contested committal hearing is underway for Alameddine, Dirani and Atai but Alou's lawyer told the court the 19-year-old would waive his right to a hearing.

Extensive police aerial surveillance footage revealed Alou, Alameddine and Dirani were under extensive surveillance in the hours before the attack.

Men allegedly spoke about weapons; took photos of ADF

Video footage also allegedly showed Alou and Dirani driving from Parramatta mosque to Jones Park in Mays Hill where prosecutors alleged they met Alameddine to obtain a gun.

In a covert recording of that meeting, Alameddine and Alou can allegedly be heard discussing a firearm.

Mr Maguire told the court Alou was upset because Alameddine had supplied a larger gun than he had asked for, and the two resolved to go to Merrylands to obtain another weapon.

"I brang the 30 cal bro. You [indistinct] wanted the big one?" Alameddine asked Alou.

"Nah, nah, nah," Alou replied.

Parramatta shooting victim Curtis Cheng, far left, and family. ( Supplied: NSW Police )

"Parramatta [indistinct] shit bro wallah [I swear by God]."

"This is bad man," Alameddine said.

"I can't help it cuz. I gotta go to Merrylands," Alou said.

The court also heard the accused men allegedly took surveillance photographs of Australian Defence Force personnel getting off a bus and shared them on the encrypted messaging service What's App.

The court heard Alou allegedly replied: "May Allah curse them all and destroy them into pieces."

The committal hearing resumes on Tuesday.