A teenager who has admitted beating his partner to death with a rock in Western Australia's Pilbara region has the cognitive function of an eight-year-old, his lawyer has told a court.
The 17-year-old, who cannot be identified for legal reasons, has pleaded guilty to murdering the woman, who was aged 18 and had given birth to the couple's second child just two weeks prior to her death in the town of Newman last May.
He has admitted striking her in the head multiple times with a rock during a drunken argument before carrying her back to his home where he lived with his great-grandparents.
State prosecutor Susan Markham told a Perth Children's Court sentencing hearing on Thursday the boy then placed his partner's body in a bin and wheeled her to hospital, telling nurses "I've taken it too far. I've hit her" before leaving.
He was arrested hours later.
Witnesses had earlier reported seeing the boy dragging his partner along the road by her hair.
The court heard that the boy, who appeared in court with a Martu Wangka interpreter, accepts he will eventually receive tribal punishment which would involve him being speared and struck with branches by elders.
A Martu elder received tribal punishment on the boy's behalf following his arrest.
The boy's lawyer Cillian Stockdale told the court his client had no previous criminal record but had come from a background of deprivation and neglect.
He started sniffing paint at age five and petrol at eight and couldn't talk until he was aged six or seven.
Mr Stockdale argued there was a causal link between the boy's cognitive impairments, including a diagnosis of fetal alcohol spectrum disorder, and his offending that night.
He said psychiatric and medical reports had indicated the boy was "operating on the level of an eight-year-old" which meant he struggled to foresee the consequences of his actions.
Children's Court President Hylton Quail said it was difficult to "disentangle" the boy's cognitive impairments from his intoxication at the time of the offending and heavy alcohol and cannabis use in the preceding months.
A psychiatric report indicated the boy, who was grieving the death of his mother, had been experiencing visual and auditory hallucinations prior to the incident.
Mr Stockdale said the boy felt deep shame and had asked to be freed on bail so that he could receive his tribal punishment as soon as possible.
"This is quite clearly a terrible and violent crime," Mr Stockdale said, noting that the victim was a slight woman who had only recently given birth.
"She should have been able to rely on (him) to look after her."
He asked the judge to consider imposing a sentence that would allow the boy out before his elderly great-grandparents died and would enable him to reconnect with his young children and complete his cultural lore process.
But prosecutors said there was no evidence for a defence suggestion that the victim had struck the offender in the head with a lump of wood prior to him inflicting the fatal injuries.
Judge Quail said it was up to the defence to prove that claim and that a trial of issues may need to be held before the boy is sentenced.
He adjourned the matter until Friday, saying it was still his intention to sentence the boy prior to his 18th birthday next week.