An extremist who gave an Islamic State salute after he was jailed for a maximum 44 years over the Parramatta terror attack, has been described as wicked by his victim's family.

Raban Alou held up a single finger and began ranting as he was led away, shouting "this is just the beginning for enemies of Islam."

Curtis Cheng's widow and son were among those in court at the time, but dismissed the outburst.

"It's just reflective of his own wicked views and I try to block it out and not acknowledge what he is trying to do which is to agitate and infuriate other people," Alpha Cheng said.

Alou, 20 pleaded guilty to aiding, abetting, counselling or procuring Farhad Jabar, 15, to commit a terrorist attack after supplying him with a gun he used to execute Mr Cheng as he left Parramatta Police Headquarters in 2015.

But Justice Peter Johnson said "the offender was not acting merely as a firearm supplier."

Instead, he was intensively involved in the planning the terror attack which he wanted carried out, and exploiting the teen gunman who had also been radicalised.

On the day of the murder, Alou and Jabar spent close to three hours together at Parramatta Mosque.

Towards the end of that time they were seen entering the female-only prayer hall which is not covered by CCTV, where police believe Alou handed Jabar the Smith & Wesson revolver.

Fourteen minutes later Jabar paused in front of a camera and gave a one fingered ISIS salute, then began the short walk to Parramatta Police Headquarters.

He shot Mr Cheng in the back of the head, killing him instantly.

The teenager shouted "allahu akbar" which alerted two senior constables who shot him dead in an exchange of gunfire.

A suicide note was found inside the pockets of his robes, it read "by the will of Allah I have come today to put terror into your hearts."

Justice Johnson said Alou's refusal to apologise to the distressed Cheng family was "devoid of basic humanity" and he noted his "perverse and dangerous views" remained entrenched.

"It is quite clear the offender remans dangerous to the general community given his views and his willingness to act directly on them."

Outside court Mr Cheng's family welcomed the sentence.

"Actions like this and hate and intolerance is really clearly not repsected or condoned in Australia and it sends a clear deterring factor for anyone who is even contemplating such disgusting acts," Alpha Cheng said.

"You cannot erase reality, we have to face it," his mother Selina Cheng added.

Police have also thrown their support behind the family.

"Whilst this was a significant sentence that was delivered today, nothing brings back Curtis Cheng, one of our own," Assisant Commissioner Mick Willing said.

"These types of sentences, if not more, will come if you undertake these types of activities." 

It is the first time anyone has been sentenced for the charge in Australia.

© Nine Digital Pty Ltd 2018