A stepfather arrested for murdering an autistic 5-year-old boy in Christchurch threatened to kill a government fraud investigator who uncovered young children left home alone a month before the deadly attack.

Police, child protection services and health professionals have all admitted failings in the lead-up to Leon Michael Jayet-Cole's brutal death.

The child was rushed to hospital after suffering a serious head injury at his Christchurch home on May 27, 2015, and died in hospital the following day.

A post-mortem examination revealed he'd suffered 44 injuries, including severe blunt force head trauma, a broken jaw, spinal bleeding and retinal haemorrhaging.

Stepfather and violent drug addict James Stedman Roberts was charged with his murder.

His murder trial was set down to begin on October 31, 2016, but he died in July that year.

The boy's mother, Emma Roberts, who is now called Emere Jayet, originally stood by her husband, telling her midwife while Leon was still fighting for his life that there was "no way [James Roberts] could've done it".

But at the first part of the inquest into her son's death last year, she says she now blamed her ex-partner.

"I now accept that James killed Leon," she told the inquest.

The inquest heard how Roberts had a "propensity for violence" towards the child, with his "pattern of angry behavior" escalating in the months leading up to the fatal incident.

Today, an Oranga Tamariki spokeswoman said that social workers had supported the family over a long period.

She accepted there had been three earlier incidents where children at the house where Roberts was a caregiver, including Leon, had been reported with bruising but Child, Youth and Family - which has since been restructured to become Oranga Tamariki – had failed to investigate further.

"With the benefit of hindsight, we didn't have a critical eye on the situation," she said.

"There were situations to step back and consider the role Roberts played in family. [I] don't believe we took those opportunities when we could have.

"James Roberts was regarded as part of the solution, not the problem."

The boy's earlier injuries, explained as accidental or unexplained, along with a police check on Roberts, "should've raised a red flag for us at that time", the spokeswoman said.

In November 2014, a social worker noted a police check on Roberts, which showed information about historic family harm and that he had convictions for violence-related offences. However, Oranga Tamariki said again, no action was taken in response to the information.

Pre-school staff and other teachers had also reported Roberts - a regular user of drugs, including heroin and cannabis - as being difficult, controlling and leaving them feeling "uncomfortable and unsafe around him".

Previous schools had advised Leon's principal to keep an eye on Roberts and he'd vowed to do so after some concerning bruises had appeared.

The Oranga Tamariki spokeswoman gave her condolences to Leon's whānau for their loss, adding that the organisation now has a "very different way of working".

Children are now at the very heart of everything they do and "never losing sight of what is best for them".

Detective Sergeant Christopher Power, who was second-in-charge of Operation Lambeth, earlier this week told the inquest that police, CYF and the Canterbury District Health Board (CDHB) all had part of the family picture but no one agency had a complete, overall view.

And, with hindsight, he accepted that police inquiries into reports of Roberts' earlier behaviour could have been handled better.

Roberts had an "inherent predilection" towards anger, violence and bullying, Power said, which was triggered by drug use or when angered at being challenged.

The family had lived at three houses in the years leading up to Leon's death and the various neighbours "could have offered quite important information" if investigators had been "steered in that direction".

The inquest, before Coroner Brigitte Windley, continues.