A senior Northland police officer is under criminal investigation after a woman laid a complaint of historic sexual abuse and harassment.

Two senior detectives have flown to Australia to interview the woman, who was the victim of a domestic violence case the officer investigated nearly 15 years ago, and other senior investigators have started contacting potential witnesses.

The Herald has chosen to not name the officer at the centre of the allegations at this time. He has not responded to messages.

A police spokeswoman late last night confirmed a criminal investigation was under way.

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"We are unable to discuss specifics at this early stage.

"Further to this, an employment investigation will be carried out. As police have the same privacy obligations when it comes to employment as any other employer, we are unable to comment any further."

The police would not confirm whether the officer would be suspended during the inquiry.

The Independent Police Conduct Authority confirmed yesterday it would carry out a separate inquiry after receiving a statement from the woman this month.

The Herald has obtained the woman's written statement to the IPCA, which said the alleged harassment began after the officer arrested her partner on domestic violence charges in 2002.

She received a few "kind and thoughtful" text messages but the complainant said the officer 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."

Even though the woman, who was in her mid 20s, began to feel "uncomfortable with the situation" she said she felt pressured to engage with him as he was dealing with the case against her partner.

In her IPCA complaint, the woman described several alleged sexual encounters.

"I didn't know what to say or do so just lay there scared and in shock.

"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."