A national investigation is under way into claims of serious delays and mismanagement by police investigating child abuse.

The Independent Police Conduct Authority announced yesterday that its five-month inquiry into delays in the Wellington police district would be extended nationwide.

Its chairwoman, Justice Lowell Goddard, said investigators had received evidence about delays and problems in other areas.

"Our inquiry will determine the extent of those delays, and what lessons can be learned to ensure that future investigations are completed effectively and in a timely manner."

The investigation began this year when it emerged that Wairarapa police had let 108 child abuse files go uninvestigated for up to 11 years, with an average delay of five years. In some cases children had been living with their alleged abusers the whole time.

Inquiries spread to the whole Wellington district and in August the authority took over the investigation from the police and began spot checks on other districts.

The Herald understands that investigators have discovered similar numbers of uninvestigated cases in other districts. One source described the problem as "systemic failure" in some parts of the country, with no proper systems in place to track cases.

The Police Association warned of widespread problems in 2006. An article in its Police News magazine estimated there were hundreds of unallocated files across the country and reported backlogs in Auckland, Tauranga, Christchurch and Dunedin.

The Herald has been told of two other districts with significant problems.

Police Association president Greg O'Connor, who has described the Wairarapa backlog as the "tip of the iceberg", said yesterday that he believed all concerned in combating child abuse - including police, doctors and social workers - would welcome the national inquiry.

The problem was partly caused by staff shortages but also a lack of focus at senior level. Administrators with no experience in police work had consistently underestimated the time required to carry out complex child abuse investigations.

"Most places are just managing to stay on top of it but when you're under that sort of pressure there's always going to be mistakes."

Mr O'Connor believed the problem was worst in smaller areas where detectives were dragged off child abuse cases whenever police needed a team for a murder investigation. Large cities with dedicated child abuse teams were generally better off.

Police Minister Judith Collins said the priority of police had to be the safety and well-being of children. "If the IPCA can help police improve their processes that is a positive step."