Justin Sims, 22, was subject to a good behaviour order and high on methamphetamine, or ''ice'', when he hatched a plan with a friend to rob a Melba service station in June.

But the plan failed when the attendant locked himself in the office, trapping the pair inside the service station and leading to increasingly desperate attempts to escape, which ultimately ended with Sims accidentally pressing the duress alarm.

He pleaded guilty to the attempted aggravated robbery when he appeared for sentencing in the ACT Supreme Court on Tuesday.

The court heard the employee was on his own and stacking shelves about 2.30am on June 24 when he saw a man come to the outlet's doors, which were locked at night.

The victim went to the counter and remotely opened the doors and watched as the seemingly intoxicated man entered.

Sims, wearing a black mask, then ran in behind the man and demanded money from the attendant.

Sims hit the victim on the left upper arm with an extendable baton when he did not reply, causing pain but no bruising.

The scared victim fled to the staff office and locked himself inside.

Both Sims and the other man attempted to kick in the door and then knocked when brute force failed.

The victim watched the pair, via CCTV, running around the store trying to escape after the sliding doors closed, trapping them inside.

Sims tried unsuccessfully to open the doors by pulling it apart, hitting it with the baton, and kicking it. The employee left the staff office to unlock the doors, but again retreated to safety when he was rushed by Sims.

Sims then climbed through the wires protecting the service counter and accidentally pressed the duress alarm in an attempt to open the doors.

He eventually fled, but his friend remained in the shop until police arrived.

Police arrested Sims after matching a palm print left at the scene.

Defence lawyer Jacqueline Agius said her client had been under the influence of methamphetamine and wanted money to buy more drugs when he committed the robbery.

She said the death of a close friend from an overdose and the break-up of a relationship had been catalyst for the offending.

She argued that Sims hitting the duress alarm showed he had little understanding of what he was doing at the time.

However, the prosecution argued the planning, although unsophisticated, showed Sims had been aware of his actions.

The court heard Sims breached a good behaviour order, which he signed when handed a suspended jail sentence for property damage, aggravated burglary, and attempted car theft in 2012, when he committed the offence.

Chief Justice Helen Murrell cancelled the good behaviour order and imposed the 18-month jail sentence previously suspended.

She also sentenced Sims to a further three years and nine months jail for the attempted service station robbery.

The judge set a non-parole period of two years and one month.

Sims will be eligible for release in October 2015.