Court documents described how the defendant had disguised himself by wearing a bandana over the lower part of his face and a hood over his head.
McFarlane was also wearing gloves.
When the victim saw him raise the hammer over his head, he immediately backed off, but his retreat was not fast enough to evade the blow which fractured his skull.
While he fled to the rear of the store, locking a door behind him, McFarlane removed 84 packets of tobacco and cigarettes – worth $3455 - which he crammed inside a backpack.
“The sale was driven by the fear there would be further robberies and his life would be at risk,” he said.
The victim had moved to another city, but still suffered nightmares of being attacked.
A report identified McFarlane’s alcohol and drug problems, but said those were secondary to his ongoing mental health issues.
He told a report writer he was hearing the voice of his dead father-in-law at the time of the crime.
His childhood was plagued by trauma, the court heard, which included exposure to violence and drugs.
Counsel Sarah Saunderson-Warner also highlighted the fact her client had been involved in a vehicle crash a year ago in which he broke his neck.
McFarlane was previously convicted of careless driving causing injury and refusing to give a blood specimen over that incident.
Young returned to New Zealand from Australia – where had lived since the age of 13 - and had “little or no support” here, which put him at a high risk of reoffending, said Crown prosecutor Craig Power.
He had two children living across the Tasman.
Like McFarlane, Young also suffered a head injury - his from a horse-riding incident 11 years ago.
He blamed his offending, which included driving convictions from an earlier incident in the Octagon, on a lack of support after his return to the country.
Both defendants were ordered to repay the cost of the stolen tobacco.