On appeal, Tuau contended that sentencing judge Peter Spiller should have allowed him discounts on three related bases: that his father provoked him; that his father had assaulted him from an early age; and that on the day of his offence he was especially vulnerable as he suffers chronic paranoid schizophrenia and was then psychotic.
The court was told that Tuau, who was 30 at the time of the offending, was temporarily living at the family's Hamilton home with his father, then aged 50, and several of his 11 brothers and sisters.
His mother had recently died and the family was still coming to terms with their loss.
On October 3 the father, who had been drinking all day, came into the kitchen where Tuau was preparing dinner.
Tuau told the court his father accused him of being a molester; an aggressive accusation that he found very threatening.
He told police he had wanted to kill his father.
Judge Spiller reduced Tuau's sentence for his lack of previous convictions and previous good character, but he did not find that his father's conduct was "sufficient provocation in the legal sense, although what your father said made you angry".
After he was sentenced, Tuau told a clinical psychologist that his father had actually threatened him with a knife; thrown him across the kitchen; choked him; forced him to do press ups for hours at night; wielded an axe near his head; and rammed his head against a wall hard enough to damage the wall.
He described his home as being like a "horror movie".
The Court of Appeal found it was highly pertinent that Tuau's father had 11 previous convictions for assaults on his other children, for which he had previously been sentenced to two years' imprisonment.
"Mr Tuau himself, as a result of the abuse he had suffered at his father's hands from an early age, was hypervigilant and fearful, more especially because he was then in a psychotic state," its decision says.
"This sustained pattern of violence to which Mr Tuau's father admitted has to supply an objective basis for Mr Tuau's claim to have been assaulted in just those ways by his father."