Trent Hapuku fatally bashed Mikara Reti in the abdomen when they were alone at a sleep out at a Flaxmere property last year.
In the High Court at Napier today, Justice Forrest Miller said Hapuku struck out once in a moment of anger, irritated that the baby had interrupted his focus on a Playstation game.
He said the court saw similar situations far too often in which a young man failed to bond with his partner's children and the home became a dangerous place, Newstalk ZB reported.
Hapuku was found guilty after a jury deliberated for three and a-half hours following a trial early last month.
Hapuku, 23, had denied doing anything to cause the death of baby Mikara, who was pronounced dead soon after arriving with Hapuku and mother Jamie Reti at Hawkes Bay Hospital in Hastings on the night of January 11 last year.
The Crown produced no evidence of any actual assault by Hapuku, but contended only he could have been responsible for the "blunt force trauma" which it said led to the splitting of the child's liver and severe internal bleeding which caused the death.
Hapuku had been in charge of the infant in a sleepout at Ms Reti's parents' Flaxmere home for much of the previous hour to 90 minutes, while Ms Reti was inside the house feeding and bathing her elder son.
Evidence was given that for much of the time, Hapuku had been focused on playing a the PlayStation game Scarface.
Calling expert evidence to counter the claims, the defence contended the injury could have been inflicted earlier and argued the Crown had not excluded all other possibilities.
