What saved David Gerald Lange (34) from a stint behind bars was the fact the man he wounded had started the fracas.
The defendant had no prior convictions for violence, too, Judge John Macdonald said, before sentencing him to nine months' home detention.
Lange had previously pleaded guilty to wounding with intent to do grievous bodily harm when he appeared in the Dunedin District Court yesterday.
The court heard he had been living at a boarding house with the victim in April but the pair had butted heads.
The victim banged on Lange's door after retrieving a claw hammer from his room.
As soon as the defendant opened up, he swung for him.
Only one of the five blows aimed at Lange struck him, on the arm, before he wrestled the weapon from his 52-year-old assailant.
The defendant hit the victim over the head several times and also punched him.
Judge Macdonald accepted Lange had not started the brawl but noted the severe injuries to the man who had.
He required 65 stitches to close his head wounds, suffered a fractured cheekbone, bleeding to the inner ear and had trouble sleeping for some time after the incident.
The victim's injuries were so serious, the only narrative the police had for the events was given to them by the defendant, who spoke to them afterwards.
Lange would serve the home-detention sentence in Auckland, counsel John Westgate said.
He was willing to undertake rehabilitation and was prepared to attend a residential programme if deemed necessary.
Lange's only previous conviction was for drink-driving in 2006.
As well as the home detention, the judge also ordered him to complete 200 hours of community work.