Ryan Taylor, 29, went to a Caltex service station in Kaleen late one night in August 2012, waiting to be let in through the locked automatic sliding doors.

He bought a pack of Tally-hos with exact change, but as he made his way out, he stood in front of the sliding doors to prevent them closing and locking.

Taylor's co-offender, Leigh Coulter, 24, then ran into the service station, armed with a tyre iron.

He jumped the counter and demanded the attendant open the till.

The terrified worker complied, and then tried to get as far away as possible from the co-offender.

Coulter took the money, but then launched a vicious assault on the attendant, hitting him in the head with the tyre iron. He then lost grip of the weapon, but continued to bash the man with his fist.

The attack was interrupted by a passer-by who was walking his dog past the service station.

He chased Coulter out of the store, using his leash to whip the man.

Taylor remained at the doors, before asking the passer-by if he could help find his dog, and then leaving.

Taylor was sentenced to two years and four months prison, with a non-parole period of one year and eight months by Justice John Burns in July last year.

But his barrister Ray Livingston appealed that sentence on Thursday, arguing the non-parole period was manifestly excessive.

Taylor had never been to prison before, the ACT Court of Appeal heard, and was suffering from depression and drug and alcohol problems at the time.

He had only a minor criminal history, and Mr Livingston argued it looked odd for someone like Taylor to be given such a lengthy non-parole period.

"A 70 per cent non-parole period is just too long, it doesn't look right," Mr Livingston said.

The barrister argued that it was clear that intervention was needed for Taylor, and said the sentence needed to be structured to allow him to rehabilitate.

"I'm not saying 70 per cent is never appropriate, but in this case it is so high as to indicate an appealable error," he said.

But the Crown, represented by Margaret Jones, said the court had rightly been concerned about Taylor's lack of insight into his drug problems, which were the reason for his offending.

Taylor had told authorities that he could manage his heroin addiction on his own, without help, something the sentencing judge John Burns described as "utterly naïve".

"He [Justice Burns] has exercised his discretion appropriately based on all the material available before him," Mrs Jones said.

She said the offender had also used heroin and cannabis on the day that he was sentenced by Burns.

Despite that, he had shown no interest in rehabilitation.

The ACT Court of Appeal has reserved its decision.