The Children's Commissioner said Moko Rangitoheriri's "abhorrent death" was preventable but agencies missed the "red flags" to investigate further.
Judge Andrew Becroft is giving evidence in the Rotorua District Court as an expert witness for the inquest of Moko Rangitoheriri.
Moko died after suffering severe beatings by his caregivers in August 2015. He was nearly 3.
Judge Becroft, the principal Youth Court judge for 15 years, has been the Children's Commissioner since the middle of 2016.
He noted the "extreme cruelty and deliberate relentless violence" which led to Moko's death had many characteristics with the abuse suffered by Nia Glassie, who died in August 2007.
Judge Becroft said there was a "dark side" to New Zealand and the country's high rates of family violence, drug and alcohol abuse, child abuse and neglect, and youth suicide are intertwined.
"All roads lead back to genuine socio-economic disadvantage," Judge Becroft said was his message about the "dark side" during his 15 months as Children's Commissioner.
Specifically referring to Moko, Judge Becroft said the responsibility for his death lay with the adults who killed him - Tania Shailer and David Haerewa.
"However, on the evidence before this inquest, it is fair to say that considerable responsibility for missing or minimising the indicators of risk for Moko must lie with the agencies who were involved with the family."
This was particularly true of the agencies working with Tania Shailer, who was struggling to cope with six children in the house and deteriorating mental health.
"These signals indicated that there were questions to be investigated around Moko's care," said Judge Becroft.
"Unfortunately, it has become apparent that the agency workers did not pick up on them."
He also echoed the evidence of an earlier witness, Dr Johan Morreau, in recommending early intervention in the lives of children when they were babies.
Three central themes emerged from the death of Moko, said Judge Becroft, which together meant social agencies failed to help his mother, Nicola Dally-Paki, solve her housing problem or investigate what was happening to Moko in the care of Shailer.
There was a lack of consultation and information sharing, said Judge Becroft.
"Multiple agencies and professionals held pieces of information which, put together in retrospect, paint a clear and tragic picture of Moko's final months and days.
"That could, and should, have prompted protective action."
Secondly, said Judge Becroft, no one appeared to look at the wider household in Auckland or Taupo.
"They focused narrowly on their own area of work and on the children they regarded as their particular clients. This meant Moko did not have anyone looking for him," said Judge Becroft.
"All the community agencies involved knew the household was under stress and that Tania Shailer had mental health problems and a history of family violence.
"Sadly, none of them thought to check on Moko's well-being."
Finally, Judge Becroft said there was a lack of understanding by agencies about poverty and hardship on Dally-Paki's ability to find a safe home for her children.
This meant Moko and his sister went to live with Shailer in Taupo.
Shailer, said Judge Becroft, used "skilful strategies" to hide what was really happening in the home.
However, she also "dropped worrying comments" into conversations with all the community agencies involved with her.
These comments included Shailer telling workers her depression was getting worse, she was stressed, not managing with two extra children and serious problems managing Moko's behaviour.
"These comments would have prompted most experienced family violence and child protection practitioners to ask more questions."
Training for frontline social workers to improve their knowledge and skills dealing with families who don't want to engage, was crucial, said Judge Becroft.
Reducing inequality and poverty was the wider social issue which needs to be tackled.
There are 10 per cent of New Zealand's children who will need significant support over a long period of time, he said.
"They do as badly, if not worse, than the same group in most western world countries."
In conclusion, Judge Becroft said Moko's "abhorrent death" was preventable and clear lessons to be learned.
He did not want to attribute blame, or make personal criticisms.
"However, it seems fair to say that if Oranga Tamariki and/or Starship Hospital had taken a different and more proactive approach to the issue of finding safe care for Moko and his sister in Auckland, the situation could have been very different."
Similarly, three agencies in Taupo - the Maori Women's Refuge, Family Works, and REAP - either missed, misinterpreted, or minimised warning signals which Judge Becroft said should have prompted further investigation.
Oranga Tamariki also failed to comply with its own seven day timeframe for a home visit following a Report of Concern.
There were plenty of "eyes and ears" aware of Moko's circumstances, said Judge Becroft.
"Yet the 'eyes' did not see and the 'ears' did not hear, nor did they trigger proper investigation about his real condition and risks."