A Sydney man has pleaded guilty to his part in the murder of police accountant Curtis Cheng outside the New South Wales police headquarters at Parramatta.

Raban Alou on Friday pleaded guilty in the Downing Centre local court to committing a terrorist act more than a year and a half after Cheng was shot dead by 15-year-old Farhad Jabar.

Redacted Crown documents allege Alou gave the now deceased teenager a .38 calibre Smith and Wesson revolver at Parramatta mosque less than an hour before the murder on 2 October 2015.

The documents also detail an alleged 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?”

Cheng died instantly after being shot in the head at close range as he left work.

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”.

“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 have also been charged in relation to the killing.

The gun 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 7 July.