Exactly how baby Maija Puhi-Duff received horrendous fatal head injuries isn't known but it's the Crown's contention it was her father who inflicted them.

The claim was made by Crown solicitor Amanda Gordon in the High Court at Rotorua when she opened the case today against Michael Donovan Duff, 42, who has pleaded not guilty to murdering the infant at Turangi on March 12, 2016.

Gordon told the jury of five women and seven men the baby's death had initially been treated as a cot death but a post mortem had revealed extensive bruising to her head, both above and below the skin and bleeding across the surface of her brain.

Medical evidence would be called to say the injuries had been inflicted by some form of blunt force causing severe trauma to Maija's head which wasn't consistent with being caused accidentally.

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"The Crown sets out to prove he [Duff] assaulted her to the head because he was the only person with her in the hours leading up to her death, he assaulted her on the head and likely caused the death of a nine-month-old defenceless baby and is therefore guilty of her murder," Ms Gordon said.

She outlined how Duff and his partner had argued in the days before Maija died and she had gone to stay with relatives. An aunt and cousin had cared for the infant on the day before she died but that evening had, at Duff's request, returned her to his care.

The jury would hear while Maija had been a bit grizzly that day, it was put down to teething. She'd taken some food and drink but rejected others that had been offered to her.

Between 5.30am and 6am on March 12 Duff had arrived at the aunt's home, saying he'd lost Maija. The aunt went to his truck and found Maija dead in the front seat.

Attempts by paramedics to revive her with CPR failed.

Gordon described how Duff told his partner he'd put the baby to sleep with him on a couch after she'd vomited up water, milk and Milo.

When he'd checked on her she appeared fine, he'd gone to sleep but woke half an hour to find she wasn't moving and he couldn't feel a pulse.

Gordon said the medical evidence to be called, including that from witnesses overseas, was very important to the Crown's case.

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In a brief opening statement Duff's lawyer Moana Dorset said Maija was a very loved child. She urged the jury to stand in Duff's shoes and look through his eyes as they heard the allegations levelled against him.

Two health professionals who had regular contact with baby Maija gave evidence she appeared well cared for.

Well Child tamariki nurse, Gael Te Rangi of Turangi, said she had counselled the infant's mother against co-sleeping with Maia because they were sleeping on a couch together.

The mother had assured her Duff wasn't sleeping on the couch too.

Questioned by Dorset she agreed the child was well loved and came form an extended family that provided excellent whanau support. She said she recalled Maija's mother was about 19 and considerably younger than the defendant who had other children from a previous relationship.

Justice Mathew Downes discharged the jury at the afternoon break for legal argument to take place in their absence.

The trial, scheduled for two weeks, is due to resume tomorrow.

Evidence will be heard from 38 Crown witnesses.