Warning: Graphic content

The death of a baby boy while in the sole care of his loving father has been described as a "rare and tragic" accident by the man's defence lawyer.

The father, who has name suppression, is on trial before a jury at the High Court at Auckland accused of manslaughter.

In the opening address, Crown prosecutor Henry Steele described the "catastrophic" head injury the boy had sustained before he died in 2017.

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By the time paramedics transported the infant to hospital and an emergency CT scan was completed the bleed in his head was 12cm long and 2.6cm wide at its thickest point, Steele said.

A large, active, or acute bleed was squashing his little brain and cutting off his blood supply, he said.

Sadly, despite the best efforts of medical staff, the damage was irreparable, Steele said.

The infant had lost 1.5 litres of blood.

The child never regained consciousness and died eight days later.

"What caused this catastrophic and fatal injury will be the central issue in this trial."

The boy had been in the sole care of his father for about 35 to 40 minutes before his dad called a Plunket helpline, a number which was saved to his cellphone, Steele said.

Relayed down the phone was that the baby boy had "gone floppy and was not moving".

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When ambulance staff arrived nine minutes later they found the baby unresponsive.

According to his father he had left the boy on a thick, soft, play mat to prepare the baby's evening bath when he heard a thud, Steele said.

The father said when he returned, the boy was holding the headphones that had been on the nearby couch, Steele said.

The accused described the boy being upset, difficult to console, and in the minutes that followed losing consciousness.

Steele told the jury the law did not require the Crown to "prove its case to a mathematical certainty".

The Crown needed to exclude the reasonable possibility of an accidental death and leave the jury sure of a non-accidental cause of death.

Steele said the jury would hear from medical experts who, though not able to completely rule out the possibility, would say it was not reasonable that the injuries suffered were accidental.

"Almost anything in medicine is possible."

But it was such a remote possibility in this case, it did not provide a reasonable explanation, Steele said.

Defence lawyer Katie Hogan said the Crown case seemed to be that tragic accidents were rare so this could not be one of them.

But tragic injuries to children who fell, even from a low level, could happen, Hogan said.

The fact it was unusual or unlikely did not rule out that it had happened here, she said.

The wrong fall, at the wrong angle, in the wrong spot could cause "devastating injuries".

It was too unlikely that this loving, calm, gentle and controlled father had in a moment of violence assaulted his own son, Hogan said.

In fact he had "always emphatically denied" doing anything to hurt his infant child.

The court heard the boy was not known to be able to pull himself up all the way to stand.

Hogan said the happy little boy was "at that stage where a child may surprise you" with what he is able to do next.

The three-week trial, presided over by Justice Cheryl Gwyn, continues tomorrow.