Ms Reti was pregnant with Hapuku's baby at the time and had since given birth to a son.
Hapuku wiped tears yet otherwise stared emotionless during statements read by his former partner, and then her mother, Selena Reti, who said she hoped Hapuku would always "see Mikara staring back at you".
Hapuku, who still maintained his innocence, was in sole charge of the infant while attempting to "clock" the PlayStation game Scarface on January 11, 2011.
Crown prosecutor Steve Manning argued Hapuku was the only one who could have inflicted the fatal blow that almost split the infant's liver in two against his spine. At sentencing Mr Manning said the blow must have been significant, would have been extremely painful and was aimed at the "most vulnerable recipient in society".
The breach of trust and failure to seek medical help for the infant were also aggravating features, he said.
Defence lawyer Eric Forster said his client may not have known the seriousness of Mikara's predicament at the time and was instrumental in seeking help for the infant in hospital.
The case hinged on the issue of timing, and whether Mikara's injury could have been inflicted sometime outside the timeframe the Crown alleged.
Justice Forrest Miller said defence pathologist Dr James Ferris had misread the pathology in claiming the rupture of the liver could have happened up to 12 hours after the injury.
"To his considerable discredit he did not abandon his theory," Justice Miller said.
He said there was an "available inference" Hapuku had lashed out in anger when the infant distracted him from the PlayStation game.
"What precisely you did we do not know ... whether it was a punch or a kick. What we do know was it was a very heavy blow.
"You can count yourself fortunate you were not charged with murder," Justice Miller said.
While Hapuku had previous convictions for possession of an offensive weapon and disorderly behaviour, he otherwise had "limited offending history".
Yet he had shown little engagement and no obvious loyalty to his partner's family, the justice said.
"This is a pattern the court sees far too often, when a woman's new partner fails to bond with her children. The home becomes a dangerous place.
"You have a child with her yourself and you should hope any future partner treats your child better than you treated Mikara."
He imposed a nine-year prison term without a minimum non-parole period, yet warned Hapuku he would struggle with rehabilitation if he continued to refuse to admit his guilt.
The officer in charge of the case, Detective Sergeant Darren Pritchard, said police were relieved the family had closure.
"This has been a tragic loss of life at such a young age," Mr Pritchard said. "It once again highlights the need for one's self-control when caring for any child. "Both families have been through this ordeal but I'm quick to point out that although Mikara's family feel that justice has been done, they will live with their loss forever."
