Aaron John McLeod, 40, has pleaded not guilty to the murder of the two-year-old son of his former partner in May 2022, who died after sustaining catastrophic brain injuries and multiple fractures.
Neither the boy nor his mother can be identified for legal reasons.
McLeod doted on her son when they first met two months earlier, the boy's mother told a NSW Supreme Court jury on Wednesday.
But the relationship soured when the two-year-old was being toilet trained, she said.
"(McLeod) would grab his arm and pick him up and sit him down and hold him," the mother said.
"(McLeod) was quite annoyed ... he believed that (the boy) was quite naughty."
The boy's mother became emotional multiple times while remembering her son's big personality and extremely active nature.
"He was very loud, he was very involved in everything, we used to call him a ... crazy hurricane," she said of her son.
The prosecution alleges McLeod went into the boy's room and inflicted serious injuries, including fracturing his pelvis and ribs, while the boy's mother was asleep in her Coffs Harbour house, on the NSW mid-north coast.
Crown prosecutor Ben Allison said in his opening address the boy's injuries could not be explained by a series of accidents in the preceding 24 hours.
The toddler fell down a set of slippery steps and banged his head during the afternoon before he died, the jury was told.
He was also found banging his head on the floor in the middle of the night in the hours before he died, Mr Allison said.
His mother testified that the boy seemed fine after banging his head and showed no physical signs of injury.
However, McLeod's barrister says there is no evidence linking his client to the boy's tragic death and no one ever saw him commit any assault on the child.
"From the outset I want to make one thing crystal clear ... nobody really knows what happened to that little boy," David Price told the jury in his opening address.
He said the boy's mother was a comfortable liar who told police different versions of what happened to her son.
The trial continues.
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