Numbers of unassigned rape cases are climbing rapidly, prompting criticism police are under-resourcing investigations into sexual violence.

The monthly backlog of adult sexual assault files waiting to be assigned to a detective has risen by 78 per cent in two years, to an average 180 each month in the last three months.

The Northland region alone had 45 sexual assault cases unassigned in September, the highest of any police district, and twice that of the next-highest area, Central.

At the same time, official data shows, reporting rates for sexual assault have remained almost unchanged.

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"It looks like there's a significant resourcing issue - not enough detectives to cope with the workload," said Victoria University criminologist Jan Jordan, the head of a current study into rape complaints.

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During a recent file analysis, researchers noted situations where some alleged victims had waited more than six months for their file to be assigned, she said.

"Those things are negatively impacting on victims who come forward expecting good treatment. And that's at odds with what police are saying about prioritising sexual violence in terms of organisational speak."

Jordan acknowledged some of the workload came from increased expectations - police now spent much longer working on each file, including liaising with victims and other agencies.

The Herald requested the adult sexual assault (ASA) data after it discovered Wellington police were secretly investigating the possibility that a serial rapist had been operating in the central city for up to seven years.

Critics said it was possible any links between cases hadn't been picked up earlier because of a lack of specially trained ASA detectives, or an overwhelming number of complaints.

Data showed some districts were clearly struggling under the weight of their caseload - Northland was the worst example, with just one dedicated ASA detective (although it did have other trained staff) and an average 43 unassigned files a month.

Auckland had 12 trained detectives but an average of 20 unassigned files a month.

However, its officers were holding an average five cases each, twice as many as the national average. It also had a huge proportion of cases in the hands of non-specialist staff.

Wellington seemed to be an anomaly - while it had the highest number of dedicated staff (17), it also had a large number of unassigned files each month.

Police Association president Chris Cahill said while jobs had been moved around to increase the numbers of ASA investigators, the numbers of unassigned files suggested more resource was needed.

He said while the Government had promised an extra 1800 staff over the next four years, most of those - 700 officers - were going to organised crime.

"One area neglected is investigators outside of that. There's also a risk experienced investigators will be lost from adult sexual assault and child abuse teams, and that puts more burden on the ones left and the less experienced investigators," he said.

Cahill said files should not be with non-specialist detectives.

The National Crime Manager, Detective Superintendent Tim Anderson, said that a case "awaiting assignment" did not mean it has not been read by an officer. Sexual assault files, like all files, were triaged to ensure that the cases requiring most immediate investigative attention were actioned first, he said.

However he acknowledged that together with an increase in reporting, numbers of cases awaiting assignment had also increased.

Anderson said within the 1800 new officers, 187 had been promised to child and adult sex abuse teams and he was confident that extra focus would help progress the files.

"We look forward to this investment boosting our capability, however, it will take time for staff to be trained in this area," Anderson said.

"We continue to encourage people with matters they want to discuss with police to come forward and speak with us in confidence."

Police Minister Stuart Nash said he was confident police were putting the necessary focus on sexual assault investigations.