The sergeant was a teacher at the Royal New Zealand Police College at the time and met the woman when he was representing police at a judicial noho marae in Turangi.
The woman worked for the Ministry of Justice.
Upon discovering they lived near each other, they decided it would be convenient to travel back to Wellington together.
"Within minutes of beginning the journey, the sergeant began telling the woman personal intimate details and stories that had sexual themes and were demeaning towards women," said IPCA chair Judge Colin Doherty today.
The woman told the IPCA that the sergeant "burrowed his hand down by my seatbelt clip, so essentially under my bottom".
He also "nudged her in a way that his arm brushed against her right breast several times".
"The Authority found, on the balance of probabilities, that the sergeant also indecently assaulted the woman by deliberately brushing her breast several times and touching her bottom."
She said the sergeant made her feel "extremely uncomfortable throughout the journey, to the point of panicking, as she became concerned about his intentions".
"(She) said she was unsure what to do, so she remained polite and friendly," Judge Doherty said in his decision today.
"She did not feel in a position to forcefully tell him to stop behaving like he was, as he was driving, they were alone on the remote Desert Road, and her mobile phone was in the boot."
Judge Doherty revealed that the pair stopped at a service station on the way and the woman saw a judge she knew had been at the conference.
"He came over and spoke to them. (She) said she considered asking him to take her home from there but 'couldn't imagine how to make that fuss'," said Judge Doherty.
"In her complaint, (she) said she felt confused, as she had spent the last four days watching (the sergeant) being a key part in the proceedings at the noho marae where everyone seemed to like and respect him.
"She said her experience of (him) was very different from other people's apparent experience of him."
The woman also told police and the IPCA that during the journey the sergeant disclosed to her that a recruit at the college had come to him concerned that he had breached police values by "having an intimate recording of his girlfriend on his work phone".
She said the sergeant told her he viewed the recording on the recruit's personal phone and later pointed the young woman out to colleagues at the recruit's graduation.
During its investigation the IPCA also considered whether the sergeant used demeaning and or sexualised language when delivering training to recruits.
"It found no evidence of this," Judge Doherty said.
"Quite apart from the indecencies, the sergeant's general behaviour was not in line with the police Code of Conduct and he did not uphold the police values of professionalism, respect and integrity."
Judge Doherty said following the car incident, a second woman came forward with a complaint about the man.
"A police recruit made a complaint alleging (the sergeant) indecently assaulted her," he said.
After that complaint the sergeant resigned from the police.
When spoken to by police and the IPCA he said the woman's claims were "rubbish".
He "adamantly denies" touching the woman, Judge Doherty said.
"He told the police officer who interviewed him: 'I tell you right now mate, I never touched her arse and never touched her tits'," he said.
"(The sergeant) told the ICPA: 'I'm a very touchy feely person, I give people hugs and kisses ... they said I touched her buttocks and her titties twelve times, that didn't happen. I've said inappropriate things, but that [saying I touched her] was really naughty."
Police College acting general manager of training Inspector Iain Saunders acknowledged the IPCA decision today.
"The actions of the officer involved were totally unacceptable and failed to represent the values and standards we expect and require of our staff," he said.
"Police treat any allegations involving the conduct of a police officer extremely seriously."
Saunders said police conducted a criminal investigation into the matter.
"(That investigation) concluded there was insufficient evidence to criminally charge the officer in question," he confirmed.
"An employment investigation was also conducted which resulted in disciplinary action."
Saunders assured that any sexual assault cases will always be treated by police "with the utmost respect, professionalism and empathy".
"Of course there can always be learnings for our officers and our organisation and we accept that," he said.
