Dwayne John James Goward, 29, rode away on a motorcycle with a man who used a fake, shortened firearm to hold up the Australia Post store on June 30, 2016 and demanded money.

Justice John Burns handed Goward a jail term of two and a half years on a charge of aggravated robbery and backdated the sentence to his imprisonment on June 30 last year, setting a 22-month non-parole period.

As Goward was handcuffed on Thursday and led away, his mother at the back of the ACT Supreme Court said "I love you, son", while other family members stood up in support of him.

Justice Burns said Goward was not the instigator of the robbery and had a lesser role but noted it was "a serious example of this kind of offending".

Goward told the author of a pre-sentence report his part in the robbery was motivated by drug use. 

Justice Burns said Goward had an "extensive criminal history", began using heroin in his teens and used methylamphetamine from an early age, but recommenced a methadone program in jail. 

He also sentenced the 29-year-old to one year and eight months' jail starting on October 30 for trafficking heroin, and 10 months' imprisonment for possessing a Taser. 

Goward, who pleaded guilty to all charges, was a "subsistence dealer" who trafficked drugs to feed his own addiction, Justice Burns said. 

Police in October 2014 searched Goward's home and found heroin and methylamphetamine.