The young radical was going to "wage violent jihad" against non-believers in an attack on Federation Square, slicing people's necks and "chopping to kill" those fleeing in terror.

After a two-month trial in the Supreme Court, a dozen jurors retired on Wednesday to consider whether he was alone in his preparations or if his younger brother Hamza Abbas, 23, cousin Abdullah Chaarani, 27, and friend Ahmed Mohamed, 25, were part of the conspiracy.

Prosecutor Nick Papas QC spent weeks building a case against the trio, relying on witness testimony alongside covert surveillance, and secret recordings from inside Mohamed's car and prison.

Mohamed and Chaarani are said to have been involved from as early as October 2016, while Hamza is alleged to have joined the plot in December.

It was alleged Mohamed saved an al-Qaeda magazine article "How to make a bomb in the kitchen of your mom" and Chaarani researched sparkler explosives.

Jurors were shown photos of evidence seized by police from the men's homes - a red tub containing more than 800 matches, the beginnings of a pipe bomb and chemicals including hydrogen peroxide that could be used in a detonator.

Other CCTV footage captured Chaarani carrying a machete into his home and the foursome meeting at Federation Square on December 21 for what Mr Papas says was a reconnaissance mission.

Witnesses included police who recalled Chaarani asking the federal officers who arrested him to "make me a martyr".

And then there was Ibrahim, the prosecution's star witness, who admitted he needed a team to pull off his plan.

The 24-year-old confessed his plot to police and pleaded guilty earlier this year.

He believed "it's fine for me to kill Australians" because Muslims have been killed in wars involving Australia.

"It's not hard to kill a person with a machete, it just takes one slice to the neck," he told police.

In the witness box he tried to take it back, claiming he told elaborate lies because he wanted to "scare the interviewers".

He portrayed himself as the leader and denied Mr Papas' suggestion that was just a lie to protect his family and friend.

Only Hamza called a witness in his defence, his sister Nabila, who backed his lawyer Felicity Gerry QC's claim he was a "fish brain" who couldn't remember what to buy from the supermarket.

"There's no point in telling him the purpose or detail, he can't be trusted," Ms Gerry said, referring to Hamza as Ibrahim's "idiot brother".

Chaarani did have extremist propaganda in his possession, was interested in Islamic State and may have believed the caliphate to be legitimate, his barrister Patrick Tehan QC conceded.

"But to download and access this material does not a terrorist make," the lawyer said, adding while it's disturbing for young men to be excited by torture that too was "not a crime".

Jurors were also told of Chaarani's desire to to travel to Malaysia.

Mohamed's interest in Islamic State goes back further than the alleged plot and jurors were shown photos of him posing with an IS flag in 2015 and heard of his attempts to travel to Malaysia.

His lawyer John Kelly SC pointed to evidence of Mohamed telling Ibrahim "no" and "not now" as evidence he had not agreed to be part of anything.

Australian Associated Press