A senior detective assured his bosses that a backlog of child abuse cases was cleared after concerns were raised by staff - until an audit two years later found 33 of the files were closed "inappropriately" in two days.

The Independent Police Conduct Authority has released the second part of an unprecedented inquiry launched when it emerged Wairarapa police had left 108 files not investigated for five years on average. In some cases, children had been living with the alleged abusers the whole time.

Justice Lowell Goddard broadened the special inquiry to cover all of New Zealand and released those "unacceptable" findings in May 2010.

Yesterday, she released a second report into the "serious failings" of police investigations of child abuse in Wairarapa and examined the response of the Wellington District and National Headquarters.

"The authority has concluded there were serious failures in the police investigation of child abuse, which must never be repeated," Justice Goddard said.

The concerns were first raised by Masterton-based Detective Sue Mackle in April 2006, who was unable to cope with more than 120 open child abuse cases. She emailed her immediate bosses and deputy commissioner Rob Pope and Detective Inspector Harry Quinn, the Wellington crime manager. Ms Mackle said she had unsuccessfully voiced her concerns several times.

"Most of my enquiry files have named offenders, some of whom are probably still abusing victims, but I am resigned to the fact that's just the way it is."

The IPCA noted Ms Mackle's "cry for help" was a catalyst for action.

Mr Quinn sent Detective Sergeant Tusha Penny to review the situation who reported in June 2006 the workload was "excessive and unmanageable" for a single officer.

Ms Penny said the files were "high risk" and recommended - among more long-term administrative changes - for a district wide operation to clear the backlog.

The report was sent to Detective Sergeant Mark McHattie, Ms Mackle's direct supervisor, to implement the recommendations. By August 2006, Mr McHattie told supervisors there were only 76 open files but 20 were "historic" and needed no further investigation. He introduced a better file management system and said other staff were "ring fenced" to help Ms Mackle clear the backlog, so extra staff from outside Masterton were no longer needed.

By September 2006, Mr McHattie told his bosses "hand on heart" the total child abuse files was just 29.

The IPCA report found police hierarchy believed the child abuse cases had been resolved. But two years later, the serious problems in the Wairarapa were once again revealed by a Wellington police district audit. Again, a backlog of more than 100 active files was discovered. The audit also discovered 46 files had been closed between August and September 2006, shortly before Mr McHattie said the backlog had been cleared. Most of those files were closed in a two-day period.

As part of the audit, Detective Inspector Shane Cotter found 33 cases were "filed incorrectly" or "inappropriately resolved" in two days. Of those, 10 were considered low risk and nine were high risk.

The two-year delay was described yesterday by the IPCA as "undesirable, unjustified and unfair" to the child victims.

Police launched Operation Hope as a short-term inquiry to fix the problem. But the 20-strong team of investigators uncovered more open child abuse files and systemic failures became apparent. More than 550 cases were reviewed, leading to 41 prosecutions.

Justice Goddard also criticised the Wellington police hierarchy for not completing an independent check of the "dramatic" reduction in files reported by Mr McHattie.

The absence of a random audit contributed to the "unreasonable" two year delay, said Justice Goddard, who also blamed national headquarters for not making such a system mandatory.

The IPCA asked Police Commissioner Howard Broad to launch an urgent national investigation. Operation Scope reviewed 2752 child abuse files and found 186 children to be "at risk" from undue delays.

APRIL 2006 Detective Sue Mackle raises concerns about 140 active files.

JUNE 2006: Detective Sergeant Tusha Penny recommends extra staff be brought in to clear case load.

SEPTEMBER 2006: Detective Sergeant Mark McHattie tells police hierarchy the backlog has been reduced. No extra staff needed.

OCTOBER 2008: An audit of child abuse cases by Detective Inspector Shane Cotter found Wairarapa still has more than 100 open cases. Operation Hope launched to clear them.

JUNE 2009: IPCA tells Police Commissioner Howard Broad to conduct nationwide audit Operation Scope.

MAY 2010: IPCA releases findings into Operation Scope. Makes 34 recommendations to implement.

MARCH 2011: IPCA releases findings into Wairarapa backlog.