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.
He ran around the corner to King St where Young was waiting in the Subaru, threw the loot into the boot and jumped into the passenger seat.
Members of the public witnessed the hasty getaway and took note of the vehicle’s registration plate.
Young drove off “well in excess” of the 50km/h speed limit towards the intersection with Gordon Rd, and sped through a red light.
He pleaded guilty to dangerous driving and robbery and was sentenced to three years and two months’ imprisonment.
The dairy owner, who spent five days in the hospital, had since sold the business for half of what he paid for it, Judge Michael Turner said.
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.