A neuropathologist has told a jury a Northland toddler died of a "catastrophic" head injury so severe the young girl could never have survived it.
More than a dozen emergency staff tended to Ariah Dawn Roberts in her Mangawhai home around 7pm on 22 August last year but the girl died shortly after.
The two-year-old had been left in the care of her mother's former boyfriend Aaron Archer, who is now on trial in the High Court at Auckland accused of murdering her.
This morning neuropathologist Professor Colin Smith, who gave evidence via AVL from Scotland, told the jury Ariah's cause of death was a blunt force head injury.
"In this case, you could have the best neurosurgeons standing by waiting to go but the outcome would have been the exact same. This is a devastating head injury."
The court heard there was evidence of bruising to Ariah's body and both sides of her head; indicating more than one impact.
Mr Smith said accidental fatal head injuries were "exceptionally rare" and required amounts of force typically seen in car accidents or falls from two-storey buildings.
"The survival in this case was very short because there was an absence of the type of pathology that I would expect to see had a child lived for 30 minutes to one hour."
Various first responders have told the court they overheard Mr Archer say he was swinging the girl around when she hit her head.
But last week ESR forensic scientist Fiona Matheson, who examined the house, told the court she did not find any unusual marks or dents in the walls.
"Nothing stood out as being immediately interesting to [suggest] there may have been an impact, punch or extended contact here," Ms Matheson said.
"The walls appeared evenly dirty; there was no indication of cleaning. There was just nothing that stood out as being unexpected or unusual."
Mr Smith told Crown prosecutor Brian Dickey he had never seen a case where a two-year-old sustained a fatal head injury in a playground situation.
"Children and toddlers fall over all the time and bump their heads all the time ... these children don't die. Fatal accidental head injuries within children are exceptionally rare.
"The vast majority of children when they fall have a mild head injury. They're certainly not immediately [dying]. They go to hospital where they're observed and managed."
Under cross-examination Mr Smith said a child hitting their head while being swung around was also a possible scenario.
"It could cause this type of injury and I've seen other cases, albeit in younger children, where that has happened. So I have seen cases with that scenario causing this catastrophic type of injury."
It's the Crown's case that Mr Archer assaulted Ariah in the 15 minutes he was alone with her; while his defence has maintained it was a complete accident.
Tomorrow the jury will hear from New Zealand forensic pathologist Dr Rexson Tse who completed Ariah's post-mortem examination, and Ariah's mother.