A Sydney man accused of being the "counter-surveillance man" ahead of the shooting murder of police accountant Curtis Cheng in 2015 acted as a lookout to make sure they were not being detected by police, a court has heard.

Mustafa Dirani, 24, has gone on trial in the NSW Supreme Court after pleading not guilty to conspiring with three other men, Raban Alou, Talal Alameddine and Milad Atai, to plan and prepare for a terrorist act and to supplying a firearm.

Mr Cheng was shot dead outside Parramatta Police Headquarters by schoolboy Farhad Mohammad, who was then shot and killed by a special constable.

In his opening address, Crown Prosecutor Paul McGuire SC told the jury about Mr Dirani's alleged role.

"He was the counter-surveillance man," Mr McGuire said.

"Whilst these important, secret and highly illegal meetings were going on [between his co-accused], Mr Dirani's role was to stand back … as a lookout to make sure they weren't being detected by the police."

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

Mr McGuire added that the accused's role was also, "to facilitate the meetings between Mr Alou and Mr Alameddine by supervising the meetings … and making sure what was going on between them was not being overheard and not being interrupted."

Mr McGuire told the jury Mr Cheng was leaving work around 4:30pm on October 2, 2015, when Fahad Mohammad selected him at random and shot him from behind.

He said Mr Alou had given the 15-year-old a .38 calibre revolver at the Parramatta Mosque earlier.

The court heard that after the schoolboy was shot dead by a special constable, police found a note inside the pocket of his robes that said he had come today "to put terror into your hearts".

The Crown will continue opening his case tomorrow and then defence barrister Mark Tedeschi QC will open Mr Dirani's case.

The trial, before Justice Peter Johnson, is expected to run for approximately five weeks.