A man who murdered his 5-month-old stepson because he was resentful the child wasn't his has been sentenced to life in prison.

William Wakefield, 32, was found guilty last month of murdering baby Lincoln Wakefield by shaking him. The sentence handed down today has a minimum non parole period of 14 years and nine months.

Lincoln was rushed to hospital on June 11 last year with fatal brain injuries. He died the following day.

Wakefield initially denied intentionally hurting the baby, but later admitted in a police interview he shook Lincoln because he wanted to hurt him, saying he was "gutted" the boy wasn't his, and that he struggled with the fact Lincoln did not look like him.

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He met Lincoln's mother when she was already pregnant.

Prior to the trial, he pleaded guilty to manslaughter, admitting he did kill the baby but denying his actions amounted to murder. He has also pleaded guilty to wounding with reckless disregard for safety, over an incident some weeks before Lincoln's death where he shook him and hit his head.

In a police interview shortly after Lincoln's death, Wakefield described the moment he fatally injured the child without provocation.

"I shook him first, not on purpose, and then I shook him again and then I shook him again ... to hurt him, I didn't mean to kill him," he said.

The Crown explained Wakefield could still be found guilty of murder despite not intending to kill Lincoln.

To be guilty of murder, it said, the killer had to intend to cause bodily harm to the victim, know such an action was likely to cause death, but consciously do it anyway.

Wakefield appeared in court this morning for sentencing, where Lincoln's mother read a victim impact statement. She said she would always picture in her head Lincoln looking at her, gurgling and cooing.

But she told Wakefield his memories would be vastly different.

"I hope you picture him on that day as you picked up a non-crying Lincoln and you looked into his big blue eyes before you brutally shook his little body between your big hands and I hope this plays over and over in your mind constantly haunting you."

The gaping hole Lincoln left would be especially felt on his birthdays, she said.

"There will be a cake with candles but no Lincoln to blow them out."

Justice Robert Dobson said the two most serious features of Wakefield's offending were the breach of trust and how vulnerable Lincoln was.

"He [Lincoln] was utterly defenceless, completely dependent on your basic humanity to treat him kindly and carefully.

"Left alone with you defenceless and with no one to monitor your conduct, he was completely at your mercy."

Baby killing by men who were not the biological fathers of their partners' children happened far too often in New Zealand, Justice Dobson said.

"It is a real blight on the way we live and it should not happen in a caring society."