Rape victims who suspect their historic cases were treated inappropriately are being encouraged to contact police and ask to have their files reviewed.

It follows a Herald investigation this month which found thousands of legitimate sexual assault cases were miscoded as "no crime" prior to 2013.

Rape Crisis spokeswoman Andrea Black said it would support any victim who wanted to go back to police to follow up their assault allegation with a specialist detective.

"We have workers that could help them," she said. "I think it's absolutely right to go back if they're not happy with how they were dealt with."

She said it could be a healing process for victims to have police believe them.

Detective Superintendent Tim Anderson, the national crime manager, said police were happy to facilitate anyone who wanted to come forward.

"Any sexual assault victim who wishes to have their case reviewed, whether it's historic, resolved or not, can request this at any time by emailing [email protected]," Anderson said. That email address went directly to national headquarters, he said.

Victims would need to supply their name, where the offence took place and a file number if they had one.

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

At least 2300 cases were coded this way prior to 2013, around 15 per cent of all adult sexual assault cases.

The practice was stopped in 2013. Last year only 3.8 per cent of adult sexual assault cases were designated K3.

Revelations about miscoding came as part of a wider Herald story 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.