The identity of a Bay of Plenty man convicted of murdering a 14-week-old baby boy, who died of severe head and brain injuries, remains a secret.

Richard Royal Uddin's killer, who cannot be identified for legal reasons, was found guilty of one count of murder by a jury in the High Court at Tauranga on Monday night.

The jury took just under four hours to come back with its unanimous verdict after hearing evidence about how baby Richard's skull "cracked like an egg" after he was assaulted and sustained non-survivable injuries.

CT scans and 3D imaging revealed multiple fractures to both sides of his skull, and the back of his scalp, and three brain lacerations with significant bleeding, the jury heard.

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Richard Royal Uddin also sustained bruising on his forehead, his left cheek and right ear.

The infant was pronounced dead on June 7 last year by a Tauranga Hospital doctor after the boy's mother rushed him to Tauranga Hospital.

The jury had two choices - guilty of manslaughter or guilty of murder.

The defendant accepted he caused the fatal injuries but denied he meant to kill the baby.

In her closing address on Monday, crown prosecutor Anna Pollett argued that from the medical evidence alone it was "inescapable" the defendant meant to do so - or at "very least he diced with the infant's life" and caused him bodily injury or injuries knowing it was likely to cause his death and carried on regardless.

Ms Pollett said the defendant had lashed out and caused "devastating" fatal injuries to the infant, who was 65cm tall and weighed just over 7kg.

She said even the defence's medical expert described the force used as "massive", despite saying one episode of severe force could explain all the injuries.

Pathologist Dr Dianne Vertes earlier told the jury injuries such as these would have been caused by the baby's head hitting "a hard surface" with high-impact violent force.

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The left depression fracture was consistent with the baby's head hitting the carpet area of a floor and the cause of the bruising could have been "a slap, a punch", or a strong squeeze.

Radiologist Russell Metcalfe likened the injuries to falling from a five-storey building.

He could not imagine the injuries being the result of just one impact, he said.

Defence lawyer Rob Stevens argued the Crown had failed to present any compelling evidence to support its murder proposition.

"Absolutely none," he said.

Mr Stevens said this was a "tragic case" where the defendant assaulted the baby.

"But at the time he certainly did not appreciate the consequences of his actions."

Mr Stevens invited the jury to find his client guilty of manslaughter.

"After hearing all the evidence, you may well be forgiven for thinking the defendant was a monster," he said.

"Even if the defendant completely lost it as the Crown suggests, you still need to be sure he had a murderous intention to kill Richard Uddin, or knew his death was likely."

"There is no evidence he was thinking at all at the time," he said.

Justice Mary Peters remanded the prisoner in custody for sentencing on August 18.