But the Crown's case was that the defendant either had murderous intent when he assaulted the infant or knew his actions were likely to lead to his death, Ms Pollett said.
"This is a trial about the death of Richard Royal, aged just 14-weeks-old.
"He didn't stand a chance at the hands of the defendant who was responsible for the devastating head injuries which caused the immediate death of this infant, " she said.
Ms Pollett said a CT scan and the post mortem investigation established the boy sustained multiple skull fractures to both sides of his head, and to the back of his skull.
Extensive blood from the infant's brain was found around his spinal cord revealing the trauma suffered by the infant, she said.
When the boy was taken to Tauranga Hospital's accident and emergency department, the infant was lifeless and not breathing.
Tauranga Hospital medical staff unsuccessfully attempted CPR and other medical interventions, the court heard.
Ms Pollett said medical evidence established there were at least two separate blows, inflicted "probably more" to the infant's head and face.
"This was no accident. This was an extreme amount of force inflicted on a defenceless 14-week-old ...[the defendant] completely lost it....and he assaulted Richard Royal with the intent to kill him, or diced with the risk that it would cause his death," she said.
The accused's lawyer Rob Stevens will give a brief opening statement to the jury today before the Crown calls its first prosecution witness.
The trial, presided over by Justice Mary Peters, was expected to take 6-7 days.