The IPCA's investigation also uncovered documents showing one of those involved, Inspect Hurimoana Dennis, broke the law in a separate incident by using his powers to influence a prosecution against his own son.
Dennis and Sergeant Vaughan Perry were charged in 2016 with kidnapping a 17-year-old boy who was dating a 15-year-old girl against his family's wishes.
In May 2015 the boy was detained at the Auckland Central Police Station, despite him not having been arrested or charged with any crime. Dennis and members of the boy's family pressured the teen to end his relationship with his girlfriend, who was 15, and go to Australia.
When he flew back in June he was detained at Auckland International Airport and put on a flight back to Australia.
At a High Court jury trial both Dennis and Perry were found not guilty of a combined three counts of kidnapping the teenager in relation to the two incidents.
But an IPCA report, released today, said it had found "on the balance of probabilities" that Dennis and Perry "unlawfully detained" the boy on both occasions.
The report said "Inspector Dennis' actions in attempting to force [the boy] to comply with his family's wishes were an abuse of his influence, power and authority as a police inspector".
He had also failed to address a conflict of interest arising from his relationship with the boy's family.
Several officers involved in detaining the teenager under orders from a superior were also criticised by the IPCA, which said they should have questioned Dennis' actions.
The report also slammed NZ Police for a "flawed" employment investigation process which "lacked transparency, leadership and coordination".
"Nobody had oversight of all the outcomes to ensure they were consistent and appropriate."
The IPCA said police should have conducted an employment investigation into Dennis' actions concurrent with the criminal investigation process.
Police had also failed to property look into the actions of all the officers involved in the unlawful detention. Only one officer had been appropriately disciplined, the report said, with the disciplinary actions later downgraded to match the leniency with which other officers had been treated.
In a second report related to a separate incident, the IPCA also found Dennis and another staff member guilty of misconduct because they had influenced the outcome of a police prosecution of offending committed by Dennis' son.
