But David Allen Will's sentence, with a non-parole period of six years, will stand because two of Justice Chrissa Loukas-Karlsson's fellow ACT Court of Appeal judges disagreed with her assessment.

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 perform the daylight heist, and the pair wielded 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, left arm, abdomen and chest.

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

Many years later, Melkie pleaded guilty to 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 cases of 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.

Will appealed against the severity of his jail term earlier this year, with his barrister Kieran Ginges arguing in February that he should have been given a discount in recognition of his assistance in the proceedings against Munro.

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

She said Will had been subpoenaed and compelled to give truthful testimony, consistent with what he had earlier told the Australian Crime Commission.

Dr Weston-Scheuber said Will was "effectively dragged kicking and screaming to the court" for Munro's trial, and he should not be rewarded for simply complying with his legal obligations.

Chief Justice Helen Murrell and Justice Natalie Charlesworth agreed in a judgment published on Wednesday afternoon, saying Will had been "most reluctant to give evidence".

They said the onus had been on him to prove he deserved the discount he was after, and he had failed to do so.

"Our primary finding is that there was no voluntariness or willingness associated with the evidence that the appellant gave at the Munro trial such as would enliven the discretion [to provide the discount]," the judges said.

But Justice Loukas-Karlsson, who heard Will's appeal with the pair, took a different view.

She said willingness to assist law enforcement authorities may be relevant to the extent of any subsequent sentencing discount, but the key issue was simply whether the assistance was given.

"There are significant public interest and policy reasons to encourage offenders to answer a subpoena and give truthful evidence," she said.

"It follows therefore that it may be appropriate to reduce a sentence on a principled basis where the evidence does in fact assist."

Justice Loukas-Karlsson found the sentencing judge had made an error by failing to consider the assistance given by Will, meaning he should be resentenced.

But given the other Court of Appeal judges outnumbered her, the appeal was dismissed and the 65-year-old will remain behind bars until at least May 2024.

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