Child, Youth and Family says it is impossible for it to know of all new births to the thousands of people its staff deal with, unless someone notifies them of a concern.
The department has admitted it did not know that a Morrinsville couple, who had two children taken into care previously, had gone on to have another child, who died from head injuries in September last year.
The death of 22-month-old Hail-Sage McClutchie is being treated by police as a homicide and is being investigated.
Child, Youth and Family chief executive Ray Smith says the toddler's older siblings were taken in to care at an early age. He says he cannot reveal any more details as it may prejudice the police case.
Mr Smith says the department has to rely on information from family, friends and professionals who are concerned about a child.
He says staff intend to improve systems to make sure that when a parent known to them has another child, they find out.