Abused men convicted of violent crimes can receive the same sentence discounts as females who have developed "battered women" syndrome, according to a recent Court of Appeal decision.

Toa Tuau, 31, was sentenced last year in the Hamilton District Court to five years and six months' imprisonment for wounding his father with intent to cause grievous bodily harm in October 2010.

Tuau was living in his family home with his 50-year-old father. On the day of the assault, the father, who had been drinking all day, accused Tuau of having been a molester. Tuau followed him into the lounge, and punched him in the head repeatedly until he was unconscious.

He then stabbed his father in the left eye with a boning knife. He told police he wanted to kill him.

The father required surgery, but fully recovered, apart from slight double vision.

Tuau told a psychologist his father had physically abused him, his mother, six brothers and five sisters, and he had been taken into Child Youth and Family care several times as a child.

After sentencing he told another psychologist, Anne Raethel, about a litany of childhood abuse his father allegedly inflicted.

The list included being threatened with a knife, being thrown across a room, held by the neck against a wall with his feet off the ground, being forced to do press-ups for hours at night and having his head rammed hard against a wall. He described his home life as being "like a horror movie".

Judge Peter Spiller, who sentenced Tuau, had a list of the father's previous convictions, which included 11 assaults on five of Tuau's younger siblings in 1998.

The father did not deny the crimes, and admitted to assaulting Tuau's younger brother from the age of five, once hitting him with a metal pipe, and once lifting him off his feet by the throat and choking him.

Tuau appealed his sentence, arguing the judge should have given him a discount because his father provoked him and had assaulted him from a young age, and because he suffers from paranoid schizophrenia and was psychotic at the time of the crime.

The Court of Appeal judges agreed. Justice Patrick Keane pointed out that in the past the court had accepted that women who suffered "battered women's syndrome" from prolonged violence were entitled to sentence discounts of up to 25 per cent. "There is no reason in principle why a man who has suffered to the same extent should be denied such a discount."

Justice Keane said children who had experienced prolonged severe abuse, live in a "climate of tension and ongoing helplessness".

The court reduced Tuau's sentence to four years and six months.

His mental illness, and the fact that Tuau and his father were mourning the loss of Tuau's mother and were especially vulnerable, were also taken into account.

Auckland University criminal law professor Warren Brookbanks said the sentence reduction made sense because there was no "monopoly" on mitigating factors in crimes.

"Of course, in this particular case there's no question he'd been subjected to some pretty harsh treatment by his father, and if he'd been a woman it would certainly have qualified as domestic violence."

Brookbanks said the court's decision added to earlier case law, and was not a major precedent.