Arthur Ronald Cottren was sentenced to 12 months' imprisonment and an 18-month community corrections order, which will start upon his release from prison. He has already served 359 days of the sentence.

Cottren was experiencing acute psychosis when he entered Albert Street's Westside Petroleum at 3.40am on August 5. The only employee on duty was a 19-year-old man.

Cottren, who was wearing a grey jacket zipped up to his nose and holding a large knife in his left hand, leaped over the counter and chased the victim who was running towards a staff room.

Cottren then returned to the counter alone and removed a cash register draw, placing it under his arm before unsuccessfully trying to open a second cash register drawer.

He yelled out to the victim, "I'm not going to hurt you, just give me the money. How do I open the till"? The teenage victim returned to the counter and helped Cottren access the second cash register.

Cottren took the cash from the register before asking the victim where the cigarettes were stored, the victim showed him and he took several packs before climbing over the counter and exiting the service station.

A total of $1000 of cash and cigarettes was stolen.

Two days later, two knives - one of which resembled the knife used in the armed robbery - was found buried in Cottren's Sebastopol backyard.

Fingerprints lifted from the scene, including from a cigarette pack he left behind, belonged to Cottren.

He was arrested in the waiting area of Barwon Hospital at Geelong the day after the armed robbery and was transported to the Ballarat Police Station where he was deemed unfit for interview due to his mental state.

Cottren was on bail at the time of the offending, charged with obtaining property by deception.

In sentencing Cottren at the County Court of Victoria on Thursday morning, Judge Gregory Lyon said a doctor had concluded Cottren was in a state of acute psychosis on the day of the offending.

He said Cottren had a long history of mental illness and had been diagnosed as having paranoia schizophrenia. Cottren believed he was constantly being watched by "gang stalkers" which resulted in him moving from town to town.

At the time of the armed robbery, Cottren had moved to live with his sister-in-law at Sebastopol and had not taken his medication for a few weeks.

The judge said one doctor diagnosed Cottren as being acutely unwell shortly after the armed robbery, but he refused to be injected with anti-psychotic medication.

"Your mental illness has been complicated by your substance abuse," Judge Lyon said.

"Your moral culpability is reduced and the predominant sentencing principles of general and specific deterrence must be moderated."

Judge Lyon said Cottren's prospects for rehabilitation were guarded and his best prospects of integration back into the community would come from community services and support.

Cottren's 18-month community corrections order includes treatment and assessment for drugs, alcohol and mental health issues, supervision and judicial monitoring. The judge directed he see Cottren by video link in three months.

Cottren understood the requirements of the court order, including not to commit an offence, and gave his permission to be placed on it.

He pleaded guilty to armed robbery and committing an indictable offence while on bail on June 3. If he did not plead guilty he would have received three-and-a-half-years' imprisonment with a minimum of two-and-a-half-years, according to the judge.

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