A victim of domestic violence has launched a formal complaint against a Northland detective for allegedly molesting her after he arrested her abusive partner.

The woman filed a complaint with the Independent Police Conduct Authority (IPCA), claiming the detective sexually harassed her while she was in a vulnerable state, according to a public police watchdog group.

In the complaint the woman claimed the detective started calling her and became hostile when she wouldn't answer his calls.

He then began visiting her house, touching her and "rubbing himself on her", according to a summary of the complaint published by the New Zealand Police Conduct Association.

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NZPCA said it was a "historic" incident as the woman had always been too scared to speak out until now.

The association published a summary of the complaint, claiming the woman was a victim of domestic violence known to police at the time of the assault.

The detective in question had previously arrested her partner, NZPCA reported.

Inspector Donna Laban, senior professional standards manager at police national headquarters, confirmed police were notified by the IPCA that a complaint has been laid.

"A formal process is now underway. We are unable to comment further at this stage," Laban said.

In 2004, former Prime Minister Helen Clark established the Commission of Inquiry into Police Conduct to independently investigate the way New Zealand Police handled allegations of sexual assault by officers.

The commission was established following reported allegations police officers may have deliberately mishandled investigations into sexual assault complaints made against other officers.

The report, released by Dame Margaret Bazley, found systemic issues and listed 60 recommendations for police, the IPCA and the Government.

In 2014, high-profile Auckland police officer Mark Gutry was cleared of a criminal sex complaint from a prostitute, but quit before an employment investigation into the incident.

The complaint of sexual violation was made by the prostitute, who Gutry paid for sex, but no charges were laid, because of insufficient evidence.