Michael Billington, 19, pleaded guilty in the ACT Supreme Court to two counts of aggravated robbery.

The court heard Billington caught a taxi from the Rivett shops on the morning of June 14.

He then led the driver into a dark cul-de-sac, covered the lower part of his face with his shirt, pulled a knife, and demanded cash.

He made stabbing motions towards the driver and said: "I'm not going to kill you. Just give me the money".

He then fled with $50.

Four days later, at about 4.30pm, Billington went to the Rivett newsagency with a hood over his head and his face obscured by a bandage.

He pulled a 30 centimetre kitchen knife from the sleeve of his jumper and pointed it at the sole staff member.

The victim handed over about $310 and the offender left.

Billington was arrested 25 minutes later in Duffy and told police: "Yeah, I was at the shops. I was the one with the knife that did the robbery."

The offender also admitted to robbing the taxi driver.

He claimed he often carried a knife after being the victim of an attempted sexual assault and had not planned to rob the cab.

The success of the first robbery then led him to plot the newsagency heist, because work had been slow and he needed money to pay for car repairs.

A victim impact statement from the cabbie said he now avoided Rivett and had become anxious about driving at night, but continued to do so because he needed to provide for his family.

A pre-sentence report found Billington had insight into the impact his actions had upon his victims and he had told the author he wanted to write letters of apology.

The author rated his risk of re-offending as low to medium, which could be further reduced by confronting his alcohol and drug use and lack of pro-social peers.

Chief Justice Helen Murrell, in October, sentenced Billington to three years and eight months jail, with a non-parole period of 18 months.

He will be eligible for release in December 2015.

The judge, in sentencing remarks published this week, said Billington was young, had no criminal history, came from a supportive family, and had good prospects for rehabilitation.

"It is important to take into account the offender's relative emotional immaturity," the sentence said.

"He has already learned a lot in the short period that he has been incarcerated, and those lessons will be reinforced by the further period that he spends in custody.

"If he does mature in custody, I am reasonably confident that he will avoid future significant interaction with the criminal justice system."