The robber, Lachlan Alan Smith, 20, faced the start of sentencing proceedings in the ACT Supreme Court on Monday.

He previously pleaded guilty to five charges, including aggravated robbery, theft, and driving a motor vehicle without consent.

Agreed facts state in July 2023, Smith followed a woman to her vehicle at the rooftop carpark of the Canberra Centre.

When she started to get into her car, Smith brandished a 12- to 15-centimetre knife, demanding the woman leave her vehicle.

The victim ran away in fear, leaving her laptop and wallet in the car.

Smith then drove at high speed before tailgating another vehicle out of the boom gate of the car park.

Several days later, Smith told police after his arrest: "It was just a straightaway thing. It was just an opportunity."

The carjacker claimed he "needed to get back south" from the City.

On Monday, defence barrister Alyn Doig said his client had spent almost 250 days in custody for the carjacking so far.

"He celebrated his last two birthdays in custody," Mr Doig told the court.

"Mr Smith is a lost boy and he needs a connection to a community. He needs strong guidance and he'll need it for a long period of time.

"He hasn't been a perfect boy, he hasn't been a perfect inmate. What seems to happen is once he's taken out of that structure he falls apart.

"This is not his first rodeo this is certainly his second time around."

Prosecutor Claire Daly argued Smith had already had "the benefit of leniency" when he was sentenced to a drug and alcohol treatment order last year.

"That opportunity was squandered almost immediately" and had been cancelled, Ms Daly said.

Justice David Mossop is set to hand down his sentence later this week.