A "cruel and vicious" assault on his intellectually disabled brother has put a Dunedin man in prison for 15 months.

Rohan Basil-James Bruce, 30, appeared in the Dunedin District Court this week after pleading guilty to assault with intent to injure, threatening to kill and possessing a weapon.

The defendant sat down with his brother in the months after the incident and apologised for his actions, saying he wished he could take it all back.

However, when he was later interviewed by Probation his stance had shifted.

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Bruce claimed to have no memory of the incident but, despite that, argued he had been provoked by the victim.

His brother should take some of the blame for the beating, the defendant said.

Judge Michael Crosbie questioned Bruce's remorse.

On November 8, he had gone to his father's home with his partner to join his brother in a drinking session.

Bruce's initial frustration was with his girlfriend and her state of intoxication but a police summary showed he found a different target for that anger.

In the early hours, the victim joined his brother in the kitchen.

While holding a large bread knife, Bruce began wrestling with his brother.

He dropped the knife and inflicted a flurry of punches, to the point where the victim was cowering on the floor.

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And yet Bruce continued.

He put the man in a headlock and hit him with more punches, shouting that he should kill him.

When the victim broke free and scrambled into the hallway, Bruce tried to hit him with a spade but was inhibited by a hanging blanket.

The defendant then threw the spade "like a javelin", damaging the victim's bike.

Police were called and found Bruce sleeping in his brother's bed.

It was not the first time he had been arrested in the wrong bed.

In 2017, after another booze-up with mates, Bruce broke into a Helensburgh home and went to sleep in a vacant bedroom.

He was sentenced to community work and supervision, to address his alcohol-fuelled behaviour.

Any counselling Bruce had undertaken, Judge Crosbie said, had not had the desired effect.

"This is a horrible assault," the judge said.

"I think cruel and I think vicious."

The victim, in a statement, said there had been similar incidents in the past but he had missed his brother while he was on bail and unable to contact him.

Bruce was granted leave to apply to have the jail term converted to home detention if he found an appropriate address.