Mr Smith, now 31, on Monday pleaded not guilty to murder but guilty to manslaughter over the death of the baby and faced the first day of an estimated four-week murder trial in Newcastle Supreme Court.

During his opening address, Crown prosecutor Neil Adams, SC, said one of Mr Smith's neighbours in Fairfield Avenue was awoken about 4.30am on February 9 by a man holding a baby.

"Help me please, something is wrong with the baby," the man said. "He is not breathing, call an ambulance." The prosecution case is that the baby boy was "in the sole care" of Mr Smith between about 2am - when they say the boy's mother left the boy uninjured at the home - and when Mr Smith raised the alarm.

The baby was taken to hospital suffering multiple serious injuries and was pronounced dead on February 10. An autopsy determined the cause of death was blunt force trauma, Mr Adams said.

Mr Smith allegedly told responding paramedics and later police that he had been attempting to settle the baby when he slipped out of his arms and fell to the ground.

But a covert listening device, installed in Mr Smith's jail cell last year, recorded him telling another inmate he had "lied" to police to protect the boy's mother.

In the recording, Mr Smith claims "the damage was already done by the mother before she took off".

Defence barrister Paul Rosser, QC, said the boy's mother had punched the baby twice in the face and twice in the body before "vigorously" shaking him.

Mr Rosser said an expert opined those acts would have caused "virtually" all of the injuries that led to the boy's death.

Mr Smith and the boy's mother had been in a relationship less than a month before his death and had spent the night of February 8 driving around Newcastle and the Central Coast to source ice and cannabis, Mr Adams said.

And Mr Smith allegedly told the other inmate in the recorded conversation that "I was off me head".

"I had drugs in my system at the time," Mr Smith allegedly said, according to Mr Adams. "They are trying to make out that I am violent, that I have done something to the baby. "I had drugs, took drugs and I did something to the baby. "They reckon I couldn't put up with the baby crying and I've harmed the baby or hurt the baby."

But Mr Smith allegedly said he was trying to "do good by" the boy's mother and offered to look after the boy.

"The baby ended up going unconscious because of what she done before she left," Mr Smith allegedly said. "And because I didn't know what injuries she had caused to him and as the night went on the injuries they started to affect him."

Mr Rosser said the defence case was that Mr Smith was "covering" for the boy's mother after he saw her deliberately assault the child.

He said Mr Smith's plea of guilty to manslaughter was on the basis that he failed to seek medical attention for the boy and was criminally negligent.

The trial, before Justice Helen Wilson, continues.

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