Her former partner Jie William Smith 31, is accused of murdering the six-month-old baby who went into cardiac arrest early on February 9, 2019 in New Lambton in Newcastle.

Smith pleaded guilty to manslaughter accepting he should have sought medical help earlier, but the Crown rejected this.

Under cross-examination on Thursday in the NSW Supreme Court, the baby's mother said before she left her son in Smith's care he was happy and healthy.

Defence barrister Paul Rosser QC submitted that was a lie and that she had just punched the baby in the face and chest twice with the side of her hand, before vigorously shaking him four or five times.

"No I didn't," she said.

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

The defence case is the baby may have experienced a seizure in "arching" back and out of Smith's arms following the mother's alleged attack, those injuries then compounded by the fall.

Smith began waking neighbours when the child stopped breathing about 4.20am, and police attended that morning.

Mr Rosser suggested the mother then called Smith multiple times to ensure he wouldn't tell detectives at the scene what she did. He responded "I am with you 100 per cent," recorded on their body-worn camera, but she denied this.

Later that morning she texted the child's father their son was on life support, that Smith was "a sick c***" and that she didn't care if he killed Smith who was no longer her boyfriend.

She sent multiple messages to Smith over the coming days telling him she loved him.

"This was affectionate texting in an attempt to keep him sweet and onside," Mr Rosser said.

"I wanted to know the truth about what happened to my son," she said.

"How would telling him you loved him get any more information out of him?" Mr Rosser said.

"Thought he might have just told me," she said.

Mr Rosser pointed to other text messages the witness sent to her sister the week leading up to the child's death, saying it showed she was struggling to cope.

"This (baby) doesn't know when to sleep and stop crying, ready to send him to his dad," read one message.

"I'm stressed so bad. (The baby) won't settle so over it. Ready to give my life up dead set," read another.

"I'm done with him ... I have no f***ing life."

"(He) just cries and I'm very very very sick. I'm going to lose it soon," she wrote the day before the baby died.

"You did lose it didn't you?" Mr Rosser said.

"No I didn't."

She also denied assaulting the child a week earlier after Smith entered the car and asked why the child was bawling his eyes out.

"Sookie f***ing kids," she allegedly said, which she denies.

Medical scans later found the baby suffered fractures to his skull, severe brain swelling, haemorrhages to the head and eyes, spinal chord trauma and extensive bruising to his legs, arms, head and neck.

The crown case is Smith murdered the child after the mother left on February 9 about 2.12am and before 4.20am when he awoke the neighbours.

The trial continues.

Australian Associated Press