Police told a young sexual assault victim it "wasn't worth the stress" for her to take her case to court, despite believing she would win.

Her file was then closed before she had a chance to respond to the decision, and the alleged offender was instead let off with a warning.

When the case didn't go to prosecution, the 22-year-old was fired from her job, where the man who attacked her was her superior.

"After I told my boss about the assault and the police complaint, they asked me to work from home because the man didn't want to be in the same room as me because I was accusing him of these terrible things," the woman said.

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"They didn't suspend him or anything. They waited to see what the police said, and then I got let go. They said it was 'restructuring'."

The Auckland woman, who did not want to be named as she still works in media, was one of a number of victims to seek a police review of her case following a Herald Investigation into sexual violence cases this year.

It revealed that 80 per cent of aggravated sexual assaults go unresolved - meaning an offender isn't prosecuted although police believe the victim.

The victim said she read the story and realised what happened to her might not have been right.

"At the time, I felt let down and it stayed in the back of my mind. But I didn't know I would be able to get my file and have a look at it," she said.

Her police file details how her colleague allegedly attacked her after a night out in December 2012. The man, in his 50s, had asked her to drive him home to his apartment because he was too drunk, and promised she could get a taxi from there.

When they arrived, instead of ordering the cab, she said the man began groping and kissing her. Because the building required swipe card access, she was unable to leave.

"He tried to rip my coat off me. I lifted my arms so he couldn't get it off me," she told police.

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"He then grabbed me ... and dragged me towards the bedroom. I think he dragged me about a metre. I was terrified, I remembered thinking he was going to lock the door and he was going to rape [me]."

Eventually, the man let her go and called a taxi. He continued to try and kiss her and stop her leaving until the taxi arrived, at one point blocking the stairs as she tried to head for the door.

When she got home, she collapsed on the floor.

A week later, she went to police and gave a formal statement. She reported the incident to her boss, and gave his contact details to police.

Documents show how the detective in charge of her case took a month before he interviewed her alleged assailant, saying he was "busy with other investigations".

After the interview - in which the man denied any offending - the detective said while he didn't believe the man's testimony, he would not be putting the case forward for prosecution.

"I feel an official warning is the best way to resolve this matter as it is not worth the stress for all involved (especially you), if this matter goes to court," he wrote in an email. "The likelihood of conviction in my opinion is strong even though it is your word against his."

Despite promising in an earlier email to discuss any resolution options with the victim before a decision was made, the detective closed the file before she had a chance to reply.

Following the review of the case in June this year, police upheld their decision to give the man a warning, although Detective Inspector Scott Beard said the wording used in the correspondence could have been more "appropriate".

The woman said she was disappointed with the result of the review, but now knew she had been correct to take the complaint.

"I was 22. I didn't know how it worked. I had thought, maybe the police knew best and it wasn't a big deal," she said. "I didn't realise I had to protect myself from police too, from them not taking me seriously."

She said looking back, she realised she had been naive not to check the detective's notes at the time.

For example, police said she had been drinking, but she only had one cup of sake. And one of the first questions she was asked was about what she had been wearing, which was irrelevant, she said.

"But there's nothing now I can do. It's done. Except, I can raise awareness and make other women not feel alone. I don't think people realise how widespread this is."

Police have said any victim who wants their file reviewed can email ASA@police.govt.nz.

Data on the proportion of decisions overturned at review was not available, however Detective Senior Sergeant Anthony Tebbutt said it was very rare for a decision not to prosecute to be changed.

Sexual assault files had extra scrutiny, he said, meaning any decision not to prosecute had to go through certain steps before it was filed.