More than 220 children who were removed from their families to keep them safe went on to be harmed in state care over a six-month period.
The new findings, the first from Oranga Tamariki's new reporting system on child harm, were described as "distressing" by chief executive Grainne Moss.
"It's really important that we keep children safe and on some occasions what this data shows is that we've failed to do that," she told the Herald.
In the worst cases, children had been raped or beaten. Several incidents led to criminal charges.
The new system is one of broadest and most detailed in any jurisdiction. It records abuse committed against a child by anyone, not just the caregiver, and in any location, not just within the child's placement. It records all incidents of harm, accidental and intentional, and ranging from over-zealous discipline of a child through to severe physical or sexual assaults.
As of June last year, there were 6350 children and young people in state care in New Zealand. Between July and September, 130 of them were found to have been harmed. Between October and December, 97 were found to be been harmed.
Some of the incidents were historical but were first reported during this period. A few children were harmed more than once. They were mostly likely to have been abused by caregivers, though many of the sexual assaults were committed by other young people or unrelated adults.
Children's Commissioner Andrew Becroft said the findings were "deeply disturbing" and "utterly unacceptable". But he said Oranga Tamariki had been bold in recording and revealing the full extent of abuse in state care for the first time.
"It's a courageous step to publish these and to not seek to hide as was often the case in the past. Now we know what we're up against, inarguably."
The new information allowed Oranga Tamariki to respond more quickly and effectively to protect children in care, Moss said. Immediate action was taken in every abuse case. In many cases the child was removed from the placement. Social workers and caregivers were sanctioned, and several criminal cases are ongoing.
One of the patterns which developed was that physical harm was usually driven by caregivers' difficulty in managing kids' behaviour, or by inappropriate use of discipline.
As a result, Oranga Tamariki was providing more training and support to caregivers.
Historic abuses of children in state care are soon to be investigated by a Royal Commission of Inquiry. The inquiry's scope is limited to abuse which occurred before 2000, though some advocates wanted a later cut-off.
Becroft said while the sector had improved significantly since 2000, the new findings "demolished" the "deluded" idea that abuse had suddenly disappeared.
"That is why we firmly submitted in our office that the Royal Commission should have discretion to investigate abuse right up until today's date - and indeed they have that discretion."
Moss said that in historical cases, many victims had complained that nobody listened to them or did anything about it.
"Whilst this report is distressing, the very strong message it sends is if you tell us we will take it seriously and deal with it immediately so we do not get something saying in 10 years' time … nobody did anything."
How to get help
If you're in danger now:
• Phone the police on 111 or ask neighbours of friends to ring for you.
• Run outside and head for where there are other people.
• Scream for help so that your neighbours can hear you.
• If you are being abused, remember it's not your fault. Violence is never okay.
Where to go for help or more information:
• NZ Police
• The Harbour, for those affected by harmful sexual behaviour
• Help Auckland 24/7 helpline 09 623 1700
• Rape Prevention Education
• Wellington Help 24/7 crisisline 04 801 6655, push 0
• Male Survivors of Sexual Abuse
• Women's Refuge: Free national crisis line operates 24/7 - 0800 REFUGE or 0800 733 843 www.womensrefuge.org.nz
• Shine, free national helpline 9am- 11pm every day - 0508 744 633 www.2shine.org.nz
• Shakti: Providing specialist cultural services for African, Asian and Middle Eastern women and their children. Crisis line 24/7 0800 742 584
• White Ribbon: Aiming to eliminate men's violence towards women, focusing this year on sexual violence and the issue of consent. www.whiteribbon.org.nz