An off-duty officer used excessive force after unnecessarily involving himself in a dispute between his friend and their former partner in Christchurch, says the Independent Police Conduct Authority (IPCA).
The officer had called for backup from on-duty officers during the 2015 dispute, realising that he had a conflict of interest in the situation.
But the police watchdog said he should have waited for them rather than involving himself in the physical confrontation, in which he used excessive and unjustified force.
The police officer received a disciplinary sanction.
The IPCA said: "Following a criminal investigation, police found there was insufficient evidence to prosecute any person involved in the confrontation.
"The Authority found there were deficiencies in this investigation and that there were potential lines of enquiry that Police should have followed up."
However, it was satisfied that the subsequent Code of Conduct investigation into the officer actions were carried out thoroughly and impartially.
Police have accepted the findings of the IPCA report.
In a statement, police said they accepted "all the findings in the IPCA's report which include that the on-duty officers attending the incident acted appropriately, including seeking assistance to mitigate a potential conflict of interest and that reviews of the criminal investigation identified potential lines of enquiry that Police should have followed up".
Police said all their teams had since been provided with conflict of interest training.