Child, Youth and Family says if it had known the violent history of a man convicted for murdering a six-month-old baby he was living with it would have intervened.
James Hemana, 31, has been found guilty in the Auckland High Court on Thursday of murdering Cezar Taylor.
The baby was shaken and dropped by Hemana on 13 July last year and died from severe head injuries in hospital 10 days later.
Hemana had previously admitted charges of manslaughter and failing to provide the necessaries of life, but maintained that he did not murder the boy. He will be sentenced on 8 March next year.
Hemana had been living with the baby's mother, Victoria Taylor, 21, at the time.
The department's chief social worker, Paul Nixon, says Hemana lied to social workers who visited Cezar Taylor's home and says they had no information that the boy was at risk of physical abuse.
Mr Nixon told Radio New Zealand's Checkpoint programme on Thursday a social worker was assigned to work with Ms Taylor, but says only later did the department learn that Hemana was threatening her and what he was doing to Cezar.
Police believe death could have been avoided
The officer in charge of the investigation says it has been a very disturbing case and Cezar's death could have been avoided.
Detective Senior Sergeant Richard Wilkie told Checkpoint on Thursday that prevention is the best defence against assaults on children.
Mr Wilkie said anyone who identified risks in a relationship to their children should come forward to government services and agencies.
Speaking to reporters outside the court, Mr Wilkie said the guilty verdict sends a strong message that children must be protected.
"We need to protect our children, our young ones. Police will do their utmost in providing a professional investigation and that working with the Crown we can put the best case forward."