A 22-year-old man laughed and refused to stand for a Supreme Court Justice as he was sentenced to a maximum 38 years in prison for his role in the murder of police accountant Curtis Cheng in October 2015.

Key points: Milad Atai last week said he was "happy" to be involved in Mr Cheng's murder

Milad Atai last week said he was "happy" to be involved in Mr Cheng's murder Supreme Court justice described Atai's risk of reoffending as significant

Supreme Court justice described Atai's risk of reoffending as significant Atai helped the man who bought the gun that was used to kill Mr Cheng

Milad Atai pleaded guilty to aiding and abetting 15-year-old Farhad Mohammad, who went onto murder Mr Cheng outside Parramatta Police Headquarters in a terrorist act.

Atai also pleaded guilty to two other charges for funding Islamic State.

In sentencing, Justice Peter Johnson told the Parramatta Supreme Court Atai showed no remorse and his risk of reoffending was significant.

Atai refused to stand for the judge and when sentenced, as he has throughout the court proceedings, and laughed when first hearing of the length of his sentence.

As he was leaving the dock, Atai spoke loudly in another language and gave a one-fingered salute which has become associated with Islamic State.

Justice Johnson said Atai's revoking of his statements of remorse — which he did in two letters sent to the Australian Federal Police (AFP) and the Crown Prosecutor in October — were a "cruel act" to the Cheng family.

"The strength and decency of the Cheng family may be contrasted with the cowardice and moral depravity of the offender," he said.

Atai assisted Raban Alou in the weeks leading up to the terrorist act, by accompanying him to meetings where he was trying to buy a gun for the 15-year-old and get an Islamic State flag made.

Alou eventually sourced a gun from Talal Alameddine on October 2 and gave it to Mohammad at the Parramatta Mosque immediately before he went to the Parramatta Police Headquarters.

Alpha Cheng speaks to media outside Parramatta Supreme Court. ( ABC News )

Mr Cheng's son Alpha said he had little hope for Atai's prospects of rehabilitation.

"Nothing will take away the pain and the trauma that our family has been put through by these twisted and depraved individuals," he said.

"Regardless of the sentence I don't think this will give them the punishment they deserve.

"We hope that justice has been served and that Milad Atai will be put away as long as possible.

"His conduct in court and his words [show] that he is still a tremendous risk to society."

'No hint of remorse or humanity'

It was revealed in the sentencing hearing last week that Atai had written letters to the AFP and the Crown Prosecutor revoking the statements of remorse he had made while giving evidence earlier in the proceedings.

He wrote that his remorse was "bulls**t" and that he was "happy" with his actions.

Atai was not present at the Parramatta Mosque with Alou and Mohammad that day as he was at work, but said in evidence would have taken the day off if needed.

"The offender said he would've been present to assist Alou if he was asked to do so," Justice Johnson said on Friday.

Parramatta shooting victim Curtis Cheng (L) and his family. ( Supplied: NSW Police )

Justice Johnson also referred to Atai's communications following Mr Cheng's death.

Three days afterwards, Atai sent messages to a closed Whatsapp group called "The Bricks Forum" saying he believed Mohammad was a martyr.

"Allahu akbar the young brother had a smile on his face and his finger up," he wrote in a message, referring to the salute which has become associated with Islamic State supporters.

Justice Johnson also referred to conversations Atai had with an undercover agent in early 2016 where he made admissions of guilt.

"He got what he deserved, he's in that environment," Atai was recorded saying, referring to Mr Cheng.

Justice Johnson said in that conversation he also suggested Mohammad was originally going to blow himself up with a bomb.

"Whether the offender was exaggerating in his reference to bombs is not clear ... what is clear is the offender was speaking in terms that celebrated the terrorist act with no hint of remorse or humanity," Justice Johnson said.