Oranga Tamariki's chief social worker says it's a complete tragedy every time a child dies.

An agency source has told Stuff at least 57 children have been killed since the child protection agency was established in 2017, and almost half had a record with the ministry before their death.

Peter Whitcombe says it's important every time a child dies through homicide - that the agency works to understand its role, and what can be learned.

"Family situations change, often there can be new relationships, there can be things that become really significant stressors. We can only act on what we know."

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