And a man who helped the inmate in the conspiracy was sentenced to five and a half years jail in the ACT Supreme Court on Wednesday.

Jackson Allred, 20, and Shane Cringle, 26, planned to rob an occasional drug dealer of cash and drugs at his home, possibly in front of the victim's partner and children.

Cringle began planning the robbery to help pay back a debt he owed after he accidentally consumed drugs smuggled in for other inmates at the Alexander Maconochie Centre.

He made a series of phone calls to Allred and his girlfriend, first identifying the target as his ex-partner, before changing it to a man living in Florey.

In sentencing the pair on Wednesday, Acting Justice John Nield described the planning as "minimal and unsophisticated".

Prison authorities were listening to the calls, and tipped off police.

Police began physical surveillance of Allred and Cringle's partner on May 17, 2011.

They watched as Allred, the girlfriend, and another two men drove to the Florey home.

Police swooped on them just as they neared their target, finding a machete and baseball bat in the boot of the car.

Justice Nield said the "timely intervention of police after clever and careful investigation" was the only reason the armed robbery had not taken place.

Both Allred and Cringle were found guilty of conspiring to commit aggravated robbery by a jury in April this year.

Justice Nield said the jury had clearly rejected the accused's evidence, in which they said the group was going to the house to buy drugs to re-sell.

The court heard Allred and Cringle had extensive criminal histories, and had abused drugs and alcohol from a young age.

Justice Nield said Cringle must receive a more severe sentence, given he instigated the plan, which was designed for his benefit. He was also older than Allred, had a longer criminal history, and had also been responsible for involving his girlfriend.

Justice Nield said their histories did not instil confidence that either could live a "drug free life" or a "crime free life".

"I cannot say either has good prospects for rehabilitation or that either is unlikely to reoffend," he said.

But Justice Nield did express some hope that the pair may be able to turn a new page in their lives.

"Neither of them, like any other sinner, is beyond redemption."

He sentenced Cringle to six years and six months, a prison term that will not start until February 2014 due to the cancellation of a parole order for earlier offences. He will be eligible for parole in February 2017.

Allred was sentenced to five years and six months, backdated to January this year.

He will be eligible for release in January 2016.