A Hawke's Bay mother is heartbroken and feels punished after her video gaming partner bashed and critically injured their baby boy, an act described as "a complete horror story".

Waipukurau man Devon Ashley Bird, 21, appeared in Napier District Court yesterday for sentencing after pleading guilty to ill-treating a child and causing grievous bodily harm with reckless disregard last month.

Judge Bridget Mackintosh sentenced him to four years and five months in prison.

The baby's 19-year-old mother was heartbroken after being "so happy and proud" when her baby was born. She said Bird's actions had left her son "fighting for his life" while "my heart was breaking".

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Her son was "taken" from her while he was at Starship Hospital in Auckland receiving treatment and is now in the care of his grandmother and CYF, she told the court.

"I loved and trusted Devon, but he has hurt the most precious person in my life."

She said facing people who asked why she allowed it to happen had also deeply hurt.

"I am being punished for Devon's actions ... I want to go back to being [my son's] mother.

"I have been robbed of a normal mother-child relationship."

Bird's offending occurred on November 1, 2013, after the mother of the then nearly five-week-old baby boy had spent the night caring for her son at their Central Hawke's Bay home.

She woke Bird about 6.30am and asked him to care for their baby, who was asleep in a bassinet, so she could rest, court documents said.

At some time between 6.30am and 10am, while the baby was in his sole care, Bird caused what a doctor said was "non-accidental trauma and extremely unusual for a five-week-old infant".

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It was also during this period that Bird, who is studying computer science at Eastern Institute of Technology (EIT), began playing the graphic video game The Walking Dead.

The game is described online as a "survival game in the midst of a zombie apocalypse".

The boy's grandmother, who called the incident a "complete horror story", arrived at the home at 10.40am when Bird told her his son had "punched himself" and suffered a bruised eye. Bird then left for class at EIT.

"She became concerned and continued to observe [her grandson] who appeared to be distressed," court documents revealed.

The baby was taken to the family GP at 1pm, before being taken immediately to Hawke's Bay Hospital.

When assessed by a psychologist, Bird said he had no independent memory of the incident and described the violent outburst as "in a dream".

Bird's lawyer Richard Stone said a defence based on automatism - an act performed unconsciously - was hard to prove but he urged the judge to consider the psychiatric report when sentencing his client.

Judge Mackintosh said Bird was a keen gamer who would often stay awake into the early hours of the morning playing video games.

Bird said he was "extremely tired and hallucinating" during the incident, the judge said.

She added Bird had become "convinced it didn't happen" and failed to tell anyone of his son's injuries earlier.

Bird also claimed he used an unusual method to relieve the baby's gas but also admitted he had accidentally fallen into a coffee table, causing a TV remote control unit to hit the boy on the head. X-rays revealed a large fracture to the boy's skull, along with more than 20 other fractures all over his body.

He was transferred to the intensive care unit at Starship Hospital on the same day he was admitted to Hawke's Bay Hospital.

Starship Hospital doctors and paediatricians described the fractures as being at "various stages of healing" and therefore were caused at different times of the boy's life.

"All the fractures were consistent with non-accidental trauma and extremely unusual for a five-week-old infant," the doctors said.

Crown Prosecutor Steve Manning said the baby may suffer long term cognitive function problems as a result of his injuries.