Sydney man Raban Alou, who's pleaded guilty after he was charged in relation to the shooting murder of police accountant Curtis Cheng, is due to be sentenced.

Alou, 20, pleaded guilty to aiding, abetting, counselling or procuring a terrorist act by Farhad Jabar, who fatally shot Mr Cheng outside NSW Police Headquarters in Parramatta in 2015.

Redacted crown documents previously alleged that Alou supplied 15-year-old Jabar with a gun at a nearby mosque before the killing on October 2.

Crown prosecutor Paul McGuire SC earlier this year told the NSW Supreme Court that Alou was lacking remorse, had extremist views and poor prospects of rehabilitation.

Three other males have been charged in relation to the killing of Mr Cheng, who died instantly after being shot in the back of the head as he left work.

The Crown previously alleged that Jabar strode back and forth while yelling "Allahu Akhbar" before being shot dead by detectives.

© AAP 2018