A verdict on whether a man who deliberately shook his infant stepson because he wanted to hurt him is guilty of murdering the child won't come today.

William Wakefield, 32, has already admitted killing 5-month-old Lincoln Wakefield and, on an earlier occasion, wounding him with reckless disregard for his safety by shaking him and hitting his head.

He has been on trial in the High Court at Wellington since Tuesday last week for murder, with the jury of eight men and four women hearing he was resentful towards the boy because he wasn't his biological child.

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

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Wakefield initially denied intentionally hurting the baby, but later admitted in a police interview he shook Lincoln because he wanted to hurt him.

In her closing address to the jury, Crown prosecutor Sally Carter pointed to lines from the interview, in which Wakefield acknowledged he didn't want Lincoln around anymore, though he didn't intend to kill him.

"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.

"I was in my own stupid world, I don't know why I did it. He's just not mine, it's hard for me to look at him."

Carter explained to the jury even though Wakefield did not intend to kill Lincoln, he can still be found guilty of murder.

"You're still guilty of murder if at the time of the act you intended to cause bodily harm to a person, you knew that that form of bodily injury was likely to cause death – real risk that death might result – and you nevertheless went on and you consciously took that risk," she said.

Wakefield said in the police interview he knew shaking a baby could be fatal. He had seen adverts and even had a pamphlet in his house about it.

Carter also pointed to evidence given throughout the trial, saying it indicated Wakefield could at times be "cruel" to Lincoln. Evidence included a photograph of Lincoln with a dummy in his mouth, with a headband wrapped around his face to keep him from spitting the dummy out.

He also put Lincoln's bassinet in the bath overnight when he was crying, despite the mother's concerns it was too cold in the bathroom.

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Carter said claims Wakefield had a "blank moment" when he hurt Lincoln did not fit.

"You don't have a blank moment and deliberately shake a child three times," she said.

"If you do what Mr Wakefield did to Lincoln on the 11th of June, then that is murder."

Defence lawyer Steve Gill said Wakefield did not realise shaking Lincoln would kill him.

"He didn't realise that because he'd done the same thing previously and the child was uninjured, on the face of it," he said.

He pointed to evidence from first responders who noted Wakefield was visibly distressed.

Gill also said evidence such as the strapped-in dummy seemed "a bit kind of sinister" but were just parents "trying different things".

"The defendant said it's to keep him happy."

Wakefield said he loved Lincoln and didn't mean to hurt him, Gill said.

The jury have been deliberating since 12.40pm, with breaks to ask questions and rewatch part of Wakefield's video interview with police. They will go home for the evening and resume deliberations in the morning.