Christopher Ross Johnson held-up the Coles Express service station on Whylandra Street about 7pm on November 10 last year.

According to an agreed set of facts, the 38-year-old opened a green Woolworths shopping back while holding a knife in his hand.

Johnson, who was wearing a pair of black sunglasses, told the attendant "righto give me all the money out of the till, just the notes" before he fled the scene with about $150 in cash.

He was arrested three hours later at the Poplars Caravan Park where he was staying.

Johnson - who has been remanded in custody since his arrest - pleaded guilty to robbery armed with an offensive weapon.

At a sentencing hearing in the Dubbo District Court on Thursday, Johnson gave evidence via video link from the Mid-North Coast Correctional Centre.

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Defence lawyer Ivy Johnson asked Johnson why he robbed the service station, which he said "homelessness, my drug addiction and need to survive as a result of having no money".

Reflecting on the incident Johnson said he was "truly sorry" to the people he robbed.

"I hope that the person that was robbed didn't suffer any psychological impacts,' he told the court.

"People that work all their lives don't expect people like me or anyone else to take their livelihoods away from them.

"I'm truly sorry for that. I just hope they can get on with their normal daily life and they don't have to suffer."

The court heard Johnson was living with schizophrenia, borderline personality disorder and anti-social personality disorder at the time of the robbery.

He had also been admitted Bloomfield, the mental health facility in Orange, and attempted to take his own life multiple times, the court heard.

Johnson discussed his childhood trauma which spiralled into a cycle of crime and substance abuse.

The court heard he was placed into foster care as a child after he suffered alcohol poisoning and chocked on his vomit while sleeping.

"I died three times in the ambulance as a result," he said.

"Then I was taken into care while the case was looked into by forensics."

It was as a juvenile living at a hostel in Tasmania, when he became addicted to drugs and alcohol.

Johnson was asked about his history of drug use, he explained "any substance is good enough" depending on what was available at the time.

"It helps me suppress my emotions and raw and negative feelings from history as an adolescent and youth," he said.

While in custody, Johnson said he struggled to access medication for his mental illnesses and was taking 150 milligrams of methadone a day.

The court heard Johnson had been attempting to get spots in rehab facilities prior to the offence but failed, and while in custody continued to request for counselling and psychiatric support, but had still not been assessed.

His last self-referral for help was in April.

"I was asked to stop sending in requests," Johnson told the court.

"They said one was enough and 'we get to you when we get to you'."

While service station attendants are legally deemed a "vulnerable person" due to the nature of their work, Judge Nanette Williams did find that Johnson's robbery was "unsophisticated and clumsy".

She accepted the defence's view that a limited amount of money was taken, and while a knife was present, it was not brandished.

Judge Williams took into account Johnson's upbringing, and need for long-term assistance to address his mental health and drug addiction issues. She also accepted Johnson's remorse which she deemed "articulate and heartfelt".

"In fact the narrative of the offender's early life is deeply disturbing and one of particular deprivation. There are few that come before the court that are as deeply disturbing," she said.

"I'm of the view the offender has reached a watershed in his life.

"He has shown considerable insight into his offending, he has hopes and plans for his future, albeit they are modest, they are in my view well thought out," she said.

With a finding of special circumstances, Johnson was sentenced to three years and two months in prison, with a non-parole period of one year and seven months.

The time served will be backdated to November 2020, and he will be eligible for release in June 2022.