On January 13, 2014, Ashworth left for work and Mitchell was left to care for their son.
"A friend of yours dropped in to see you for about 10 minutes. Honour was awake, on the couch and happy," sentencing judge Justice Nicholas Davidson said.
"He was a happy, healthy boy who was not suffering from any sickness, other than a slight cough."
Later in the night, at 7.23pm, Mitchell phoned 111.
"You said Honour must have fallen off the couch and that he was not breathing," Justice Davidson said.
Paramedics rushed Honour to Southland Hospital after he was found unresponsive.
A CT scan later revealed subdural haemorrhaging to his brain, and an ophthalmologist found that he had severe retinal haemorrhaging in both eyes, the court heard.
Honour, his mother and Mitchell were flown to Starship Hospital in Auckland during the early hours of January 14. But the infant died that night.
A post mortem concluded that the direct cause of death was ischemic encephalopathy. A forensic pathologist said that a fall from a low couch, as Mitchell had explained to police, was unlikely to account for the degree of trauma.
The pathologist sought a second opinion, which also concluded that abusive head trauma was the most likely cause of Honour's death.
"For a long time no charge was laid. Suspicion by the police was not proof. You were the only person with knowledge of the facts," Justice Davidson told Mitchell.
When police formally interviewed Mitchell on January 24, 2016, he denied violently shaking his son.
However, he said after waking to Honour screaming he noticed his son had gone limp and "stuff [was] coming out of his nose and mouth, but it was not milk".
"You said that you lifted him to your shoulder and gave him a bit of a shake. You insisted that the shake would not have hurt him. You said you were trying to get stuff out of his mouth," Justice Davidson said.
Mitchell was interviewed for a second time by police on July 20, 2016, and when confronted with overwhelming medical evidence finally admitted to shaking his son.
He told police he had attempted to feed Honour milk in an effort to calm him, but when it failed he held his baby under his arms and shook him.
"That was when Honour's head went limp," Justice Davidson said. "This was the first time you admitted shaking him in this way, two years after he died."
When reading a victim impact statement from Ashworth, Justice Davidson said, she "had everything to live for as a hard-working mother of two beautiful children".
"She has been stopped in her tracks by grief. The discovery of how Honour died, in her words, 'ripped her soul to pieces'."
The judge said Honour's grandfather's description of bringing the infant's "wee body back home" captured the "cold reality of the consequences of what you did".
The boy's grandmother also described her pain after initially believing Mitchell's story.
"She feels the weight of guilt that she did not protect her family from you," Justice Davidson said.
"Her instincts warned her about you, and she says, a piece of her has died. She would prefer that you never touch another baby or family."
The judge said everyone in the court was "moved and affected by what happened".
Justice Davidson added the gravity of Mitchell's offending was reflected in his son's "extreme vulnerability".
"He was a little baby ... He could not have been a more vulnerable victim. You were his father and there is no greater breach of trust than by a parent to his or her child."
Mitchell's lawyer, John Westgate, emphasised his client's actions came "out of the blue" and Mitchell had no history of such offending.
However, Justice Davidson said Mitchell's failure to own up to what he did aggravated his offending.
"You concealed the truth and it took two years to uncover that while the police investigated and Jenna and the whanau were left in doubt. You then challenged the medical evidence."
Mitchell will serve two years and six months in prison before he is eligible for parole.
