The incident took place in Pukekohe on May 12, 2018, when the officer's brother was involved in a physical altercation with his neighbours.
The officer was visiting his brother when a car crashed into a fence outside his brother's house.
The pair went outside to see what happened, as did his neighbours - a couple, and their son.
The brother mistakenly thought the son had damaged the fence and a fight broke out between them.
Police were called and the brother was arrested and charged with two assaults,
threatening behaviour before being released on police bail.
The officer went back to work and told his supervisor in Tuakau that his brother "had been a dick" and started a fight.
He told his supervisor he would be a witness for the defence and recognised it as a conflict of interest.
The brother pleaded guilty to assaulting the son but denied the charges of threatening him and assaulting his mother.
The officer, who worked in Pukekohe, elected to be a witness for the defence and completed a written statement and gave evidence in court, authority chair Judge Colin Doherty said.
"The presiding judge rejected the defence evidence and found that there was a clear attempt by the officer, and his brother's wife, to protect his brother by minimising his actions."
Doherty said based on its own investigation, the authority found that the officer lied in his evidence to the court to minimise his brother's behaviour and protect him.
"Additionally, he breached the police conflict of interest policy by failing to adequately inform his supervisor of his involvement in the trial."
Further, the Pukekohe prosecutor should have transferred the file to Manukau and filled out a conflict of interest form, he said.
The authority also found the prosecutor's demeanour in dealing with the victims "was at odds with the Police Prosecution Service statement of policy and practice".
"Our own independent assessment of the evidence drew us to the same conclusion as the court as far as the officer's intentions to protect his brother were concerned.
"The differences in accounts between witnesses are such that there is no way either party could simply be mistaken and we were drawn to the further conclusion that the officer lied to the court in an attempt to protect his brother."
The report also found that there were "different versions of events" as to what information the officer provided to his supervisor and whether he continued to keep him sufficiently updated throughout the process.
The supervisor said he recalled the officer mentioning the assault and that he'd told him he did not see any assault on the female, and some of the people involved had assaulted his brother.
The supervisor later called the acting area commander to advise her of the situation.
He said he thought he was aware his brother was bailed to the officer's address while on bail.
The authority found he breached the conflict of interest policy and should never have used police time to file his formal written statement, which was given as evidence in the case.
The authority was critical of the supervisor's lack of knowledge around the conflict situations particularly given he was a supervisor in a small community where conflicts were likely to arise.
As for the prosecutor, the victims had complained the prosecutor initially in charge of the file "became rude" throughout their interview.
However, when spoken to by the authority, the officer said he didn't "become" rude, as he is "…normally rude and abrupt from right at the beginning".
The authority found he should have transferred the file to the Manukau team sooner than the Friday before the Monday in February that the case was due to start.
