Two-year-old JJ Lawrence was killed despite a worried aunt contacting Child, Youth and Family just a month before his death.
Yesterday, on the first anniversary of the boy's death, Joel Loffley was found guilty of murdering JJ at their Orakei home by striking him so hard in the abdomen that he split his liver and pancreas in two.
Child, Youth and Family disclosed details of their involvement with the family after the verdict.
Operations general manager Marama Edwards said JJ's aunt, Katie Lawrence, contacted the agency after JJ suffered a second broken arm a month before his death.
Ms Lawrence raised "concerns about supervision of JJ and possible neglect".
However, the agency said the report did not indicate JJ was at "immediate risk" and medical professionals said the broken arms were "consistent" with the explanations given by JJ's mother, Josephine Lawrence.
Paediatric expert Patrick Kelly testified at trial that after his death JJ had the tell-tale signs of abuse - clusters of numerous bruises in areas other than the extremities where toddlers normally bruised themselves.
Ms Edwards said a social worker visited the house several times over the next couple of weeks but could not find JJ or his mother.
"We now know adults in the home were lying to agencies about people's identities, the relationships between people in the house, and who was living there."
She said CYF only learned that Joel Loffley - a known criminal who was then on a community detention bracelet - went by that name three days before JJ's death.
"JJ died days later while our investigation was still under way."
Josephine Lawrence was pregnant to Loffley at the time of JJ's murder.
It is understood the new baby has been put into care.
Social Development Minister Paula Bennett declined to comment on the case because Loffley had not been sentenced.
She issued a statement last night saying: "The suffering of any child is heart-wrenching. I am pleased that someone has been held to account for the brutal murder of Baby JJ."
Police investigation head Detective Senior Sergeant Kevin Hooper said outside court that the family did the right thing in alerting authorities.
"Unfortunately, time ran out for JJ," he said. "It's sad, we didn't get there in time."
The Lawrence family burst into tears and gripped one another's hands but then ran from the court almost immediately after the foreman announced the verdict.
JJ's uncle, Richard Lawrence, later said the family were happy that the jury had reached a decision but it was "sad for all the families involved".
He said the whanau were having a memorial for JJ on Monday, probably near where he is buried at Whatuwhiwhi in Northland.
They were having a "debrief" last night and would comment after the sentencing on February 14.
The CYF revelations showed that, despite their good intentions, the family were unable to overcome a mother's desire to stay with her son.
The wider Lawrence family recognised something was wrong after Loffley moved in and JJ started suffering mysterious injuries.
He had bruises and suffered two broken arms. Loffley told Lawrence he "may have" fallen on the boy when they were "playing rough".
Friends spoke of seeing JJ, once a happy and outgoing little boy, now fearful and reserved in Loffley's presence.
Family had already tried to separate JJ from the harmful environment in Auckland.
He had been taken to live with aunt Katie Lawrence in Tauranga after his first broken arm, but Josephine brought him back to live in Orakei. The second broken arm was allegedly from a fall down some stairs.
As in the Kahui case before it, the JJ defence rested on casting suspicion on the mother to create doubt that it could have been her that murdered her son.
Josephine Lawrence admitted she was an alcoholic who smoked P and cannabis and had not taken JJ to hospital for eight days after he broke his arm.
She said she was "wasted" and did not want her son taken from her.
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