Child Youth and Family put a three-year-old girl at unacceptable risk by providing misleading information about her father, a Family Court judge has said.
The girl had been living with her mother, who was taking drugs and in an abusive relationship with a violent boyfriend.
However, Child, Youth and Family (CYF) repeatedly - and wrongly - told the court it was the girl's father who was violent.
The incorrect information added a five-month delay to the man gaining the care of his daughter, who by that stage had gone to live with her grandmother.
In his decision, Judge Anthony Walsh dismissed the CYF reports about the father as unsubstantiated and unverified.
"CYF acknowledged it had provided incorrect information about the father being violent ... Understandably the misleading information distressed the father and compounded his frustration particularly with CYF," Judge Walsh wrote.
CYF has apologised but the man is now suing the agency to recoup more than $10,000 in legal costs and pay for therapy for his daughter.
The agency said yesterday that the father could seek a review by the Chief Executive's Advisory Panel if he was not satisfied.
But the father told RNZ the panel would be a waste of time.
The father had told CYF 13 times over three years he was worried about his child's safety.
In 2015, Social Development Minister Anne Tolley replied to a letter he sent her, saying she had been assured by the agency's chief executive that a thorough investigation "has not found concerns requiring action to be taken".
Documents show social workers dismissed his concerns, saying he appeared to be acting maliciously and would be considered abusive if he did not stop.
They also questioned his mental state and said there was no evidence to support his allegations.
The man's sister, who was praised by the court for helping arbitrate between the father and the mother, said social workers told them if her brother made any more complaints they would put him on supervised access.
In a later court hearing the mother admitted the father was right to be concerned - her boyfriend ended up charged with raping the mother at the family home in 2015.
In a ruling issued last December, Judge Walsh said he was satisifed the father had "legitimate grievances" about the reliability and accuracy of CYF's information.
"This non-disclosure of information should not have occurred. Errors of fact were repeated in CYF reports," he wrote.
"The father and [his sister] raised concerns about the accuracy of the information; in particular, how aspects of notifications about the father became facts without being verified.The observations about the father being violent to women were not supported by independent and verified data.
"Regrettably, this effectively caused approximately a 5-month delay in resolving ... care arrangements. Such delay caused unacceptable further risk to [the girl's] welfare and best interests."
For over a year, CYF also did not reveal a positive drug test by the mother which showed sustained use of cannabis.
On the same day the drug test came back positive, a senior CYF manager wrote to the man denying his request for an independent review.
Hospital staff had sent a report of concern to CYF in 2013 about the girl having a pulled elbow, with no explanation for how it happened.
CYF also had a 2015 record of the police contacting them worried about the toddler.
A CYF file note days later said there were concerns the girl had been exposed to drug use and was at risk of physical violence
In a May 2016 Family Court hearing the mother accepted she could not provide day-to-day care for her daughter, acknowledging her past drug use and a previous violent relationship.
A psychologist had found she loved her daughter but was not stable enough to look after her.
A psychologist reported to the court that the father had no history of aggressive behaviour or drug abuse, was a good carer for the girl, and this compared with multiple concerns about the mother's ability to keep the girl safe.
Time was short at that point, the psychologist told the court. "I feel as if we're absolutely at the edge of ... what can be retrieved."
The agency said in a statement last night that it would take a close look at its practice, as it always did if the Family Court raised concerns.
It refused to give an interview, citing the need to protect people's privacy.