But a prosecutor has argued David Allen Will's testimony does not entitle him to a discount, saying he was "effectively dragged kicking and screaming to the court" and compelled to give truthful evidence.

Will, 65, was the brains behind the May 2004 robbery of an armoured van outside the Mawson Club, conceiving the plan and paying a Chubb employee $15,000 for inside information about the vehicle's movements.

He recruited Mark Anthony Munro and Sam John Melkie to carry out the daylight heist, and the pair were carrying guns when they confronted two armoured van guards during an afternoon cash pick-up.

Munro shot guard Kevin Matangi, who survived, with a sawn-off shotgun at close range, spraying pellets across the victim's face, arm, abdomen and chest.

The robbers ultimately made off with a total of $151,995 in cash.

Many years later, Melkie pleaded guilty to a charge of aggravated robbery and was sentenced to nine years in jail.

Munro, however, pleaded not guilty to the same charge and to an allegation of intentionally inflicting grievous bodily harm.

When Munro stood trial, Will gave evidence against him and he was found guilty and locked up for 15 years.

It was not until after the proceedings against Munro and Melkie had concluded that Will himself was charged, and eventually convicted of aiding, abetting, counselling or procuring an aggravated robbery.

He was sentenced by Justice David Mossop in 2018 to 10 years and 10 months behind bars, with a non-parole period of six years.

In the ACT Court of Appeal on Wednesday, Will's lawyers argued that the judge incorrectly failed to apply a discount for the offender's "assistance to the administration of justice or law enforcement authorities".

They also claimed the sentence was "manifestly excessive" and inconsistent with Justice Mossop's findings.

Barrister Kieran Ginges, instructed by solicitor Tom Taylor, said in his submissions that while Will was compelled to testify in the Munro trial, the 65-year-old chose to give truthful evidence.

He noted that the judge in that trial, Acting Justice John Nield, told the jury that Will's evidence was perhaps "paramount" to the Crown case, which would possibly have collapsed without it.

"The jury's verdict in the Munro trial reflects its acceptance of [Will]'s evidence beyond a reasonable doubt and the significant role it had in Munro's conviction," Mr Ginges said.

The barrister said that when Justice Mossop came to sentence Will, the judge "was bound to consider the assistance given by [Will], and his failure to do so amounted to error".

"The error occasioned a miscarriage of justice in [Will]'s sentencing, warranting this court's intervention and resentencing," he said.

Prosecutor Kylie Weston-Scheuber disagreed, however, arguing that the discount Will was seeking on his sentence should be reserved for people who had given evidence voluntarily.

She said Will had only cooperated after being hauled before the Australian Crime Commission and compelled to answer questions about the robbery truthfully.

When Will was then subpoenaed to testify in the trial of Munro, Dr Weston-Scheuber said, the prosecution had a transcript of what Will had told the commission and he was warned that he could be charged with perjury or contempt of court if he lied.

She said that, as a result, Will "had no choice" but to give the evidence he did against Munro.

"He was effectively dragged kicking and screaming to the court," Dr Weston-Scheuber said.

She said it would be "an absurd result" if a person who was legally compelled to give evidence could later rely on that "so-called assistance" for a discount on their own sentence.

Dr Weston-Scheuber argued that Will's appeal should be dismissed.

A full bench of the Court of Appeal reserved its decision on the matter.