Thousands of reported sex offences disappeared from official statistics over a 20-year period after police inappropriately coded legitimate assaults as "no crime", a Herald investigation has found.

In cases such as those with limited evidence; or where victims were intoxicated; or consent was unclear, instead of recording the incident as a "K6 - crime reported" police would label it "K3 - no crime".

Documents show about 15 per cent of reported rapes or other sex crimes were being miscoded as recently as 2013, leading to the impression there were both fewer cases in total, and that police were solving more cases than was true.

Because police also used the incorrect code for the small minority of fake complaints, some officers conflated the two, resulting in the mistaken belief that a huge swathe of women were liars.

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"That confirms everything we've seen in the community," says National Collective of Rape Crisis spokeswoman Andrea Black.

"It's left so many people traumatised. People end up taking sides, saying: 'She lied because police aren't investigating, she's crazy, she's chaotic.'"

Read more: Why New Zealand rape victims are left in limbo

Information released to the Herald shows it took almost 10 years after academic Jan Jordan first highlighted the issue for police to commission their own report and move
to stop the practice.

That was despite the fact police engaged an external consultant to review Jordan's work in 2004, and that her findings were later expanded upon in a paper for the Ministry of Women's Affairs in 2009.

The police review, completed in 2013, was released to the Herald only after the Office of the Ombudsman intervened. It reviewed 279 of the roughly 40 per cent of adult sexual violation offences coded K3 that year, and found 107 - or 38 per cent - were miscoded.

Based on these figures, there would have been at least 2300 victims of aggravated sexual assault in the 20 years before 2014 whose cases were written out of official data.

If non-aggravated sex assaults were also included - which they were not because of complications with the data - the number would be even higher.

Police redacted the main finding from their review. However, the recommendations were clear.

"Police must cease the practice of K3-ing sexual violation offence reports unless there is credible evidence to show that on the balance of probabilities, an offence did not occur," it said.

National crime manager Superintendent Tim Anderson said the K3 report was never finalised, but police were no longer coding sex crimes they couldn't resolve otherwise as "no crime".

Only 3.8 per cent of adult sexual assault cases were designated K3 in 2017, he said.

Additionally, police had introduced a new recording system, with more nuanced codes. They had educated investigators about compliance with reporting standards.

Detective Superintendent Tim Anderson said police were no longer miscoding sexual assaults. Photo / File
Detective Superintendent Tim Anderson said police were no longer miscoding sexual assaults. Photo / File

Anderson noted that sexual offences were under-reported, and that police would be concerned by the publication of any material that might inhibit reporting further.

"All of our investigators are very passionate about this crime type and work very hard for and on behalf of victims and their families," he said.

The revelations about the use of K3 codes are part of a wider Herald investigation into violent sex crimes, which found 80 per cent of aggravated sexual assaults go unresolved - meaning an offender isn't prosecuted although police believe the victim.

For the crime "male rapes female over 16", the unresolved total was 85 per cent, the highest since records began.

Advocates says prosecution rates are unlikely to change until there is a reduced focus on victim behaviour, and more onus on the offender.

Parliamentary Under-Secretary to the Minister of Justice Jan Logie said she would take recommendations on reform later this year.

"I am not going to defend the status quo. We need to do better, and I am committed to doing better," she said.