The Association of Social Workers says it is unacceptable that Child, Youth and Family gave a complex case to an inexperienced worker with little support.
The department has admitted failings in the case of a woman who was sentenced on Tuesday in the High Court in Auckland to a prison sentence of two years and 10 months after her one year old son drowned when she left him in a bath last November.
CYF acknowledges it did not carry out final checks before returning the baby to his mother and the file was allocated to an inexperienced social worker.
It says the social worker was not properly supervised because of high workload and staff shortages.
The Association of Social Workers says that is not good enough.
The Public Service Association says it does not want to comment. It says only that it will be take up the issues of workload and staffing with management, which is not a discussion it intends to start in the public domain.
Children's Commissioner John Angus says Child, Youth and Family makes difficult judgements about complex cases all the time.