Child, Youth and Family says it wasn't told two-year-old JJ Lawrence was at risk of being harmed when it was contacted about possible neglect a month before his death.
JJ's stepfather Joel Loffley, 29, was found guilty of JJ's murder by a jury at the High Court on Wednesday, one year to the day after he died in his home in Orakei, east Auckland.
JJ died after being struck in the stomach with such force that it split his liver and pancreas in half.
CYF operations general manager Marama Edwards says it heard from JJ's aunt Katie Lawrence just over a month before JJ's death.
It was the first time CYF had been notified about JJ.
"Katie Lawrence's notification raised concerns about supervision of JJ and possible neglect," Ms Edwards said.
"This report did not indicate JJ was at immediate risk of harm."
Ms Edwards said they were told JJ had suffered a broken arm twice, but that "medical authorities told us that the explanations were consistent with the injuries".
She said a social worker visited the home several times in the next month but couldn't 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.
"Child, Youth and Family only learnt on Wednesday, November 9 (five days before JJ died) that Joel Loffley went by that name."
Police investigation head Detective Inspector Kevin Hooper told Radio New Zealand he believed CYF was following the right protocols.
He praised Ms Lawrence for contacting CYF, and said the help from JJ's family and people who knew Loffley was key to the jury's guilty verdict.
"People were prepared to come forward and tell the jury what they knew and in my view that was the key to success in the case."