The NSW government does not know how many reports of sexual abuse against children in state care it receives.
It decided that compiling totals of alleged abuse incidents was not a "reasonable" use of resources.
Government lawyers rejected a formal request for the numbers under freedom of information laws as NSW prepares for an Upper House inquiry into child protection services.
"It is not possible to auto-extract the information," a Department of Family and Community Services spokesman said.
Jacqui Reed, chief executive of the Create Foundation that represents children in care, called the lack of transparency "absolutely outrageous".
"They should have this information at their fingertips," Ms Reed said. "The data is very important to us. It's a way to keep governments accountable."
The department assumes the legal responsibilities of a parent for nearly 6000 children.
The Labor opposition had formally asked: "How many reports of sexual or physical abuse against children currently in the care of the Department of Family and Community Services has the Department received in the year 2014/15?"
It asked the same question about non-government care providers, which look after about 7500 children.
The legal unit of FACS rejected the requests, citing "certain limitations" of the department's IT systems.
In response to questions from Fairfax Media, a department spokesman then said: "While we do capture [the information requested], our ability to represent it over a year is limited."
The spokesman also said the department could not "readily determine" if an alleged abuse incident took place while the child was in state care or before. It is running a trial that would fix this for new reports of children found at risk of significant or actual harm.
"How can the government determine where resources are most needed if accurate statistics about abuse aren't readily available?" opposition community services spokeswoman Tania Mihailuk said.
"The minister must intervene and make this information available immediately. What has he got to hide?"
A spokeswoman for Community Services Minister Brad Hazzard said the department made its own decisions on information requests but was improving data collection.
The NSW Ombudsman, which tracks allegations against out-of-home carers, recorded 609 last year, of which 23 per cent were verified.
But the Royal Commission into Institutional Responses to Child Sexual Abuse last year exposed deficiencies in record keeping by state governments.
Children are responsible for roughly half the sexual abuse of children, experts estimate, but these cases are not recorded by the NSW Ombudsman unless the victim has a disability or staff are said to have neglected their responsibilities.
The last national survey of child abuse reports, by the Australian Institute of Health and Welfare in 2014, found more than half the reports were not investigated.
Ms Reed, of the Create Foundation, said the NSW government often took months to supply "the most basic information" about child welfare.
"When we do get it, it's often inaccurate," she said.
The Upper House inquiry into child protection services will examine "the capacity and effectiveness of systems, procedures and practices to notify, investigate and assess reports of children and young people at risk of harm."
Labor has said it would appeal the department's decision not to provide the totals.
