More than 70 children and young people were abused while in Child Youth and Family care last year and no central records are being kept on the abuse.
Social Development Minister Paula Bennett revealed the information in Parliament today in response to a question from the Greens' Holly Walker.
Ms Bennett said that of the 71 cases of abuse - 30 were by CYF-approved caregivers and the rest by "third party'' caregivers.
"Thirteen have been prosecuted by police, the others have not. Even though they were substantial forms of abuse - they weren't at a level police were able to prosecute,'' Ms Bennett said.
"I am assured by the department that all of those children are now being cared for and are safe and all carers who were known to be abusing children are no longer in a position of care.''
She said the figure equated to less than 1 per cent of all caregivers. International figures ranged from 0.5 per cent to 7 per cent.
She said she asked CYF in 2010 to review cases where children placed in CYF care had been further abused or neglected.
"It, quite frankly, horrified me that we didn't know how many children and young people in our care were abused and re-abused,'' she said.
The review was carried out by a social worker and took seven weeks.
Ms Walker said the minister must take immediate steps to ensure the collection of national records of abuse of children in CYF care.
"For an agency that's in the business of child protection it is astonishing that it didn't see fit to collect data on children in its own care who were subsequently abused in that care,'' she said.
Ms Bennett said it would require a major IT overhaul to collect the data centrally and the Ministry of Social Development was looking into a system upgrade.
"CYF central database can report on the number of findings of abuse, it cannot automatically identify and report on the number of cases of children or young people who are abused during the time they were in care,'' she said.
"Unfortunately we're not in every home and able to make sure every child is cared for at the level that we would like them to be. What I can promise is that they are followed up, there are processes in place, they are prosecuted whenever possible and those children are not left in unsafe circumstances,'' she said.