Forster was convicted of indecent assault and sexual violation in March 2020 and sentenced to six years in prison. He was paroled in April this year.
The IPCA report did not look into Foster's attack on his colleague, but it did examine other events of that evening, many of which had been captured by CCTV cameras at the motel occupied exclusively by police officers for the Waitangi deployment.
It found that some of the behaviour on display in outside areas of the motel breached the code of conduct which police are supposed to adhere to even while off-duty.
This included officers binge-drinking from the baton-like vessel, stripping to their underwear, taking selfies while drinking next to marked police vehicles, and making derogatory comments.
One senior sergeant was filmed removing his shorts, sitting down and spreading his legs, exposing his genitals for about 90 seconds. That officer was suspended and later resigned before he could face a disciplinary process.
The 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 Treaty Grounds in the following days.
"There were several examples of unprofessional and inappropriate language and behaviour that evening," the IPCA said.
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."
But the IPCA report also looked at three other incidents involving off-duty police in the same year.
These were at police-sponsored sports events in March and September 2019. Although the officers were off-duty, travelling in unmarked or private vehicles and paying for their own accommodation, the code of conduct still applied.
Police at a basketball tournament in March participated in an initiation ceremony which involved drinking six shots in quick succession and then sculling another drink.
A female officer who had taken part in this was later asked to leave a nightclub because she was severely drunk.
She was escorted back to the motel by another officer, and an incident happened which led to a sexual assault investigation. However, no charges were laid.
At the Police Winter Games in September, an officer who had been eating and drinking in a bar was found passed out in a toilet cubicle at 3.30am.
When roused, he abused and punched the bar owners. He later pleaded guilty to assault and resigned.
In a separate incident, two officers in a relationship had a physical fight in a bar. One of them was charged with assault but this charge was withdrawn after the officer participated in the diversion scheme.
The IPCA found there were "appropriate courses" to deal with such breaches of the code of conduct, either under employment processes or the criminal law.
"However, there is a clear area of risk that needs to be properly managed," the report said.
"In principle, off-duty officers are entitled to socialise and drink alcohol, as long as this is done responsibly and in accordance with the code of conduct."
