Aaron John McLeod is standing trial accused of murdering the two-year-old son of his then-partner in the early hours of the morning in May 2022 at her house in Coffs Harbour, on the NSW mid-north coast.
Neither the child nor his mother can be identified for legal reasons.
The 40-year-old's barrister told the jury in his opening address on Tuesday there was no evidence linking McLeod to any physical abuse of the young boy either on the night he died or in the two months beforehand.
"From the outset I want to make one thing crystal clear ... nobody really knows what happened to that little boy," David Price told the NSW Supreme Court.
"Nobody has admitted any serious assault against (the boy), either directly to police or to any other person."
The jury heard on Monday that the toddler had hit his head on some wooden steps the afternoon before his death and was found banging his head on the floor in the middle of the night in the hours before he died.
However, prosecutor Ben Allison said these incidents were trivial and could not have caused the catastrophic brain injury found to have caused the boy's death.
Mr Price contested the the prosecution's version on Tuesday, saying it was open to the jury to accept the possibility they caused the tragedy.
"(Mr Allison) described these as 'relatively trivial events'. It's the defence position that they were a bit more than that," he said.
A blood-stained T-shirt belonging to McLeod found in the home's washing machine after the boy's death was the result of the accused helping move the boy after he began bleeding during CPR, Mr Price said.
He told the jury both McLeod and the boy's mother had their phone calls and messages intercepted by police in the days after the tragedy, but no evidence linked either of them to any serious assaults on the toddler.
However, McLeod's barrister did cast the boy's mother as a comfortable liar who gave multiple different accounts to police of what happened to her son.
"(This case) is not just as neat as the Crown laid out yesterday, you've got your work cut out for you," Mr Price said.
The trial continues.
1800 RESPECT (1800 737 732)
Lifeline 13 11 14
Men's Referral Service 1300 766 491
Kids Helpline 1800 55 1800 (for people aged 5 to 25)
16°C