Oranga Tamariki got it wrong when it decided not to investigate nine out of 14 reports of possible abuse or neglect.
An internal review has found shortfalls in the way these reports of concern were initially accessed - and two-thirds should have been referred for further action.
A report of concern is when a member of the public or someone like a teacher or doctor notifies Oranga Tamariki that a child or young person may be at risk of harm.
If Oranga Tamariki is worried about a child's wellbeing, an assessment takes place or, in more serious cases, an investigation is carried out.
In 2020, close to 81,000 reports of concern were made to Oranga Tamariki and about half were referred for further assessment or investigation.
In its November 2020 report, the Independent Children's Monitor noted a "lack of visibility" over initial decision-making, particularly in cases where no further action was taken.
Oranga Tamariki had made changes in response to this and had started sampling decisions, to assess the quality of decision-making, the report said.
The monitor said this was a "positive step", as Oranga Tamariki "need to have confidence that decision-making is robust".
From this sampling, Oranga Tamariki reviewed 69 reports of concern made between April and June 2020, where a no further action decision was made.
Of those decisions, 14 were classified as relating to possible incidents of abuse or neglect.
When those cases were examined further, Oranga Tamariki said it appeared the decision not to take further action was wrong in nine cases.
The Monitor said the local Oranga Tamariki offices followed up with the tamariki and whānau involved.
"Considering the number of cases where the decisions were inaccurate, the Monitor has asked Oranga Tamariki what steps it is taking to improve decision-making accuracy," the report said.
In its official response to the Monitor's report, Oranga Tamariki said it would continue to undertake reviews of a sample of no further action decisions, to ensure decisions are accurate.
Assistant Māori Commissioner for Children Glenis Philip-Barbara said Oranga Tamariki must do better.
"I would expect in the first instance that Oranga Tamariki will be taking a close look at every no further action finding for a complaint of abuse, and satisfying themselves and ensuring that everything that can be done and should be done, has been done," she said.
Aotearoa New Zealand Association of Social Workers kaiwhakahaere Braden Clark said individual social workers were not to blame.
"From what we are hearing from members, social workers are over-worked and they have got very high caseloads, they have not got enough support, there is not enough staff," he said.
"I think this is probably more a reflection on the systemic conditions that are happening within Oranga Tamariki, than individual social workers' practice."
Clark said social workers wanted more training, supervision and resources, so they had the time and expertise to make the right decisions.
In a statement, Oranga Tamariki said how it responds to allegations of harm for tamariki in care, including how it assures it is making the right decisions when concerns are raised, is part of a range of continuous improvements it is making.
"We report on these to the Independent Children's Monitor as part of the external monitoring requirements and this information will form part of the Independent Children's Monitors Annual Report later in the year," the ministry said.