The partner had his in-car camera recording as he drove through Wollongong in the early hours of November 30 last year, with 20-year-old Lachlan Mitchell beside him, making persistent requests for money. The victim refused to hand over any cash, saying "I can't give you any more money because you already owe me a lot".

Video played to the Wollongong District Court on Wednesday showed Mitchell producing a large knife when it became apparent the money wasn't coming.

"Give me $100 or you are going to get this through your neck right now," Mitchell said. "I'm gonna stab ya, just give me $100."

The victim eventually relented and gave two $50 notes to Mitchell, who then left the car.

He later pleaded guilty to charges of robbery while armed with an offensive weapon.

The court heard Mitchell had a significant criminal history and was on parole for property crimes at the time of the offending.

Representative from the Director of Public Prosecutions, Jacqueline Azard, said Mitchell he spent most of his adult life in prison. "Yes, he's young, yes he's had mental health problems, but there are concerns in terms of his prospects of rehabilitation," he said. "They're not non-existent, but they're very low," she said.

But defence solicitor Patrick Schmidt said Mitchell "wasn't a person that society gives up on". Mitchell was 14 years old when he was involved in a car crash that killed his stepfather and permanently confined his mother to a wheelchair, the court heard.

He freed his younger step-siblings from the wreckage and rendered First Aid to his mother before help arrived. Later he was diagnosed with PTSD. A psychologist's report to the court said his pre-existing mental health problems worsened and he turned to drugs.

The court heard Mitchell was paid damages from the accident but that news of this had leaked in prison, making him a target. He had been subjected to six assaults while in prison, and was now allowed only limited amounts of respite each day, for his own protection, Mr Schmidt said. "His payout is being monitored by a trust, however ... he's being stood over on a regular basis," he said.

"Not only that, his mother and grandmother have been contacted and told, 'if you don't pay us, this is going to happen to Lachlan'."

Judge Christopher O'Brien found the offending was the by-product of a "rampant drug addiction". He said Mitchell would need a signifiant period of supervision in the community after his release from prison.

Mitchell was sentenced to three years' imprisonment with a non-parole period of 18 months. With time served, he will become eligible for release on January 29, 2022.