Wellington police have apologised for the "unacceptable" delays and insufficient investigation into child abuse cases.
The admission followed an announcement by the Independent Police Conduct Authority that it was investigating delays in a number of cases in the Wairarapa.
Police were also conducting their own inquiry under the authority's direction.
A Masterton District Court judge last week criticised police over a two-year delay in prosecuting a 45-year-old man for assaults on his two daughters.
Authority chairwoman Justice Lowell Goddard said today the case reflected wider issues with the police response to child abuse cases dating back at least to 2006.
Acting Wellington Police District commander Superintendent Gail Gibson said police would cooperate fully with the authority's inquiry.
"Investigations involving vulnerable child assault victims are complex, difficult and at times protracted," she said.
"These and other factors including our file handling practices, high workloads and resource allocations have contributed to an unacceptable time delay dating back in some cases of several years."
Police acknowledged there were problems and notified the authority in June this year.
Ms Gibson said "robust new systems" had recently been implemented to deal with child abuse investigations.
"Timely investigations involving child abuse are a top priority for us.
"There has been a shortfall in the way some cases have been handled and for that we apologise. We have failed some child abuse victims, their families and the community. We've also let ourselves down in not providing the level of service we would have like to meet our own and public expectations."
A backlog of 108 files in the Wairarapa was identified last year during the planning process for a new Wellington District Child Protection Team .
"It became clear to us...that a backlog of files existed in parts of our district," Ms Gibson said.
"The backlog was compounded by investigators being drawn off their day-to-day work to support other serious crime investigations."
She said the Wairarapa CIB was no different to many others in that respect and had meant some complaints, "were not as thoroughly investigated as we would have liked."
"This is not an excuse, it has been the reality. We have moved quickly to implement changes in the way in which we allocate files, have undertaken an independent review, improved our processes and file supervision ."
Wellington police had been working to ease the backlog with a special operation, investigating more than 100 files since December and making 30 arrests.
The Child Protection Team was expected to be fully operational later this year with 10 specialist investigators.
Inquiries by both police and the authority would be asking who was aware of issues and what had been done in response.
"This would include questions of possible misconduct or neglect of duty, compliance with policies and general instructions, the adequacy of existing policies and questions of supervision and oversight," the authority said in a statement.
Police would also be auditing child abuse files in other regions to ensure delays were not a widespread occurrence.
