Employment probe into alleged inappropriate behaviour of police staff after CCTV footage reviewed as part of investigation of sex assault by one officer against a colleague.

A group of off-duty police officers were allegedly drinking heavily and partying in the hours leading up to one staff member allegedly sexually violating another in the group.

The alleged inappropriate behaviour of a number of police officers into the early hours of February 5 is being investigated as an employment matter, with one officer already stood down.

The Herald on Sunday understands there were around 20 police staff, including sergeants and a senior sergeant, staying at a motel.

Some were allegedly seen drinking beer from a police baton-shaped glass, stripping off their clothes to jump into the pool and walking through the drive-through lane of a nearby McDonald's.

The police proactively issued a press release on Monday after the Herald on Sunday contacted the motel manager, who declined to comment.

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"The allegations relate to inappropriate behaviour that occurred over the course of the evening of February 4 and the early hours of February 5, 2019, while the officers were deployed outside their district," Assistant Commissioner Bill Searle said.

"The New Zealand Police's code of conduct applies at all times, regardless of whether staff are on or off-duty.

"Police treat any allegations relating to potential misconduct by police officers seriously.

"We are unable to comment further at this time due to the ongoing employment investigation."

The Herald on Sunday can reveal the internal investigation is directly linked to a criminal investigation after one of the female officers made a formal complaint that she was sexually assaulted by a fellow staff member.

Detectives investigating the criminal complaint reviewed security camera footage from the motel, which captured some of the alleged behaviour in question.

It is unclear whether the internal investigation was triggered by the footage alone, or an actual complaint from the motel staff or guests.

But following the criminal investigation, one officer was charged with the sexual violation and indecent assault of one his colleagues.

The 28-year-old appeared in the Auckland District Court this month and was granted interim name suppression

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At the time, the police issued a statement which confirmed the charges related to two incidents in the early hours of February 5.

But there was no mention of the separate employment investigation into the alleged misbehaviour of the wider group until a fortnight ago.