Baby Karlos Stephens and his twin brother were born six weeks prematurely - 10 months later Karlos was dead from a massive brain bleed.

At the trial into his death the prosecution contends that bleed was caused when Shane Claude Roberts, 60, assaulted the infant while he was in his care at Rotorua at the end of November 2014.

Roberts pleaded not guilty to Karlos' murder when he went on trial in the High Court at Rotorua this morning.

Karlos Stephens, 10 months, died from head injuries in Rotorua in November 2014. Photo / File
Karlos Stephens, 10 months, died from head injuries in Rotorua in November 2014. Photo / File

Crown solicitor Amanda Gordon told the six men and six women on the jury that medical evidence established Karlos' injuries weren't consistent with a non-violent death.

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"The Crown says it is the defendant [Roberts] who assaulted the 10-month-old and he did so regardless of whether Karlos would die or not and is therefore guilty of his murder," Gordon submitted.

She emphasised it was not known exactly how the injuries were inflicted.

She outlined how Karlos' mother Pamela Stephens, who also had a 4-year-old son and three older boys, struggled to cope with her newborn twins.

Tipu Ora believed she was suffering from post-natal depression.

Roberts, whom Stephens met some months earlier, offered to help her with the twins, assisted by his ex wife, also the mother of twins.

"That caused her some relief to get back on her feet," Gordon said.

In July 2014 both became the twins' principal caregivers, Roberts referred to himself as the boys' father and also told some people he was her biological father, which was untrue.

Roberts was living in Alison St, but spent a lot of time with the twins at his former partners' Homedale St home.

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Gordon described how Robert's ex wife went to Christchurch for her daughter's graduation leaving him in charge of the twins.

At Robert's invitation their mother was staying at Alison St which had no power – when her cell phone went flat she had no means of recharging it.

The jury would hear that when Roberts brought the babies to Alison St on the afternoon of November 29, 2014, Karlos was grizzly and clingy and Stephens asked Roberts to take him to a doctor. He said he'd take him to Lakes Care after he'd been to see someone, Gordon contended.

Roberts didn't return until 7.30am the following day, arriving in a flurry and banging on the door saying Karlos wasn't breathing.

There was what Gordon described as a "heated discussion".

She wanted to take the child straight to hospital but Roberts wanted to call an ambulance.

Stephens' wishes prevailed and en route to the hospital she cradled him in her arms.

"She was willing him to live." Gordon said.

Emergency staff were unable to revive Karlos who had no obvious signs of injury. A post mortem the next day found bleeding across the surface of his brain, he also had extensive eye bleeding.

"He ultimately died from a lack of blood and oxygen to the brain," Gordon said.

Roberts was arrested in July 2018.

The trial, which is before Justice Matthew Muir, is set down for two weeks. The Crown is to present evidence from 36 witnesses.