Binge-drinking off-duty police officers breached their code of conduct at Waitangi in 2019, with a senior sergeant exposing his genitals, and others stripping to their underwear, an investigation has found.

The inquiry by the Independent Police Conduct Authority (IPCA) said there was a "general lack of professionalism and respect for the public-facing environment and context of the deployment" among some officers who would be policing the Waitangi Day commemorations at the Treaty Grounds in the following days.

"There were several examples of unprofessional and inappropriate language and behaviour that evening," the IPCA said.

These included the use of an improvised drinking vessel fashioned to look like a PR24 riot control baton.

Assistant Commissioner Richard Chambers said police had already acknowledged that the behaviour of the officers' involved was "completely unacceptable and extremely concerning.

"Their behaviour is not representative of New Zealand Police culture."

The IPCA report covered a number of incidents which occurred in 2019, some of which resulted in criminal charges.

It did not deal with that of a constable who was later found guilty of raping a sleeping colleague at the Northland motel on the night of the binge-party.

Jamie Anthony Foster, 31, was convicted in March 2020 and jailed for six years after a jury found him guilty of indecently assaulting and sexually violating the woman.

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He was later granted parole.

- Ric Stevens, Open Justice