Man pleads guilty to handing over the gun that killed Curtis Cheng

On Friday, 19-year-old Raban Alou admitted to committing a terrorist act, more than a year and a half after Mr Cheng was shot dead by Farhad Jabar.

Redacted Crown documents allege that Alou gave Jabar a .38 calibre Smith and Wesson revolver at a Parramatta mosque less than an hour before the murder outside the NSW Police headquarters in October 2015.

Raban Alou, 19, provided the gun that killed Cheng. Photo: 7 News

They also detail a conversation with a female associate the day after the attack in which Alou mentioned "beautiful footage".

"It makes me so happy," Alou allegedly said during the conversation.

"See how he drops? Did you see how much fight he had in him?"

The Crown documents tendered to the court allege Jabar and Alou were alone in the female-only section of the mosque for about seven minutes before Alou left on the afternoon of the killing.

It's alleged Jabar then prayed in the mosque's main hall before changing into a black robe, called a dishdasha.

On the way out, he looked up at a CCTV camera and held up his right index finger in a salute used by Islamic State militants.

Mr Cheng died instantly after being shot in the back of the head at close range as he left work on October 2.

Murdered NSW Police finance worker Curtis Cheng. Photo: AAP

The Crown alleges Jabar then strode back and forth while yelling "Allahu Akhbar" before being shot dead by NSW Police special constables.

A note found in his pocket talked about "unbelievers" and proved the killing had been a "religiously inspired act of violence in support of IS", the papers allege.

"Know your weapons are nothing compared to what we have," the note allegedly said.

"Know that you all are being watched 24/7."

Three other men, who are all accused of supporting Islamic State ideologies, have been charged in relation to the killing.

The gun used to kill Mr Cheng was allegedly delivered to Alou and a co-accused at a western Sydney park by another man riding a bicycle.

Alou is due to next appear in the Supreme Court on July 7.