But the man's barrister submitted that the mother was "utterly untruthful" and had deliberately inflicted injuries on her own son.

The Crown and the defence gave closing addresses on Thursday at the Newcastle Supreme Court trial of Jie William Smith.

The 31-year-old has pleaded not guilty to murdering his then partner's six-month-old son between 2.14am and 4.20am on February 9, 2019 in New Lambton.

The Crown has rejected the Newcastle man's guilty plea to manslaughter made on the basis that he failed to seek medical attention, knowing the mother had seriously injured the baby.

Before the closing addresses, specialist forensic pathologist Professor Johan Duflou gave evidence for the defence.

He said it was "entirely possible" that Smith's version of what happened fitted in with the presentation of the child's condition when taken to hospital.

This involved the mother punching the crying baby before vigorously shaking him around 2am, leaving him unsettled and fractious, with dim eyes and refusing to eat.

The mother has denied assaulting her son.

Smith says that after she left he tried to settle the distressed child and feed him a bottle but he wriggled out of his arms and dropped him.

Prof Duflou said an earlier shaking and punching incident could have led to a deterioration in brain functioning and to seizures which could be a reason why the baby fell out of the man's arms.

Under cross-examination, he agreed all of the baby's injuries could have been caused at the same time.

Prosecutor Neil Adams SC submitted that the baby was in the sole care of Smith when he suffered blunt force trauma.

While the Crown could not say how or precisely when the injuries were inflicted, the mother told police and the jury her baby was unharmed when she left.

Smith initially said he dropped the baby, but when medical experts said this alone could not have caused the injuries the prosecutor said he later blamed the mother.

His claim he lied to police at first about the mother's assault to protect her was an attempt to escape liability, Mr Adams said.

His decision to "take on this role of gallantry" was not taken off his own back, but - according to Smith - when she asked him not to say anything.

But Mr Adams said they had only been seeing each other regularly for a few weeks, he had sent her "over-the-top" texts and didn't care at all about her feelings.

Four months later he told police about the alleged assault, a move the Crown described as a recent invention and "inherently implausible".

"It doesn't make sense he would say nothing about this matter and lie to protect her in those circumstances."

Smith's barrister Paul Rosser QC said the jury had seen his client "warts and all", noting his drug use and his use of "extremely foul language".

"He is not boyfriend of the month, not someone you would be happy for your daughter to bring home," he said.

But prejudice by jurors should be put aside, as should sympathy sparked by seeing photos of the dead child.

Smith didn't have medical knowledge and cannot but have felt responsible at first, thinking the child died after he dropped him.

He didn't know then that really serious injury could come from shaking.

But Mr Rosser said the mother had "almost an addiction to untruthfulness" and listed many lies told by her.

"She is utterly untruthful, utterly, utterly untruthful."

Justice Helen Wilson will give legal directions to the jurors on Friday before they retire to consider their verdict.

Australian Associated Press