An investigation is under way into allegations a Northland detective sexually abused a woman he was meant to be helping escape from an abusive partner.

The written complaint - which police have confirmed to the Herald they are aware of - said the abuse began after the officer arrested her partner on domestic violence charges in 2002.

The woman, who had been living in an area north of Auckland at the time, said she'd laid the complaint now because she was "finally strong enough to do it".

Independent Police Conduct Authority [IPCA] case resolution manager Sarah Goodall confirmed a complaint against a senior Northland officer had been laid.

"I can advise that the authority has been making background enquiries and expects to have a decision on how the complaint will be addressed by the end of the week."

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Police National Headquarters Senior Professional Standards Manager, Inspector Donna Laban, said police were aware of the complaint.

"Police have been notified by the Independent Police Conduct Authority that a complaint has been laid and a formal process is now under way. We are unable to comment further at this stage."

The complainant said at first it was just a few small texts from the senior officer to see how she was doing.

"I thought he was kind and thoughtful and just doing his job in the beginning."

But the woman said things evolved from there and he became a "bit too friendly", buying her flowers, asking her out for dinner and giving her a nickname.

"He asked me out for dinner and wanted me to wear a red or black dress. I declined and felt a bit weird about it. He then arrived at the house with wine and pizza."

She said the text messages and calls "just to chat" continued after this.

Even though the woman began to feel increasingly "uncomfortable with the situation" she said she felt pressured to engage with the officer as he was dealing with her case against her partner.

On one of his visits, when she began to feel unwell after drinking some wine, she went to lie down.

The complainant described the officer coming into her room, turning out the lights, getting undressed and joining her in bed.

The woman, who was in her mid-20s at the time, said he began rubbing his naked body against her back.

"I didn't know what to say or do so just lay there scared and in shock. After a while he began to shake and shiver then stopped.

"I was just frozen and silent. I had gone through two violent relationships and was still sorting out the mess of those.

"He was so much older than me, was meant to be helping me and I was relying on him. I was disgusted but a part of me felt like I couldn't say anything because he was the police and I needed his help at the time."

She went and described in detail two other incidents where she said the officer made sexual advances on her.

In the last incident she detailed how the police detective forced her down on the bed, and performed oral sex on her.

In the complaint she also recounted how the officer appeared to turn up at her family's home unannounced and at one time appeared to be lying in wait in an unmarked police car.

"I was so scared of being near him in case he touched me again or did anything to me, but I was also so terrified that he was the police and I needed the police so couldn't do anything."

The woman eventually moved to Auckland two years after she first met the officer in an effort to avoid running into him again.

She described feeling unable to turn to the police for help or support and so, after another violent altercation with her ex-partner, made the choice to flee the country with her children to keep safe.

"It took me years to recover from everything that happened to me," she said. "I have changed my life, but this has always played on my mind.

"He was meant to be helping me and protecting me and he helped destroy me at the time and did things to me that no policeman should be doing to any person."