Police in Christchurch have accepted faults were made in the case of an off-duty officer who hit a teenager in 2011.
The authority says police were wrong not to formally interview or prosecute the officer who pursued and hit the 17-year-old with his baton in June 2011 for damaging his letter box in the suburb of Fendalton.
Authority chair Judge Sir David Carruthers said on Thursday despite the serious nature of the officer's actions, a criminal investigation was never undertaken and this was a departure from the proper process.
The authority also found that the decision not to prosecute the officer because it was not in the public interest to do so was made without proper consideration of the prosecution guidelines.
Canterbury District Commander Superintendent Gary Knowles said he is satisfied there was never any deliberate attempt to protect the officer or to minimise the seriousness of the offending.
Mr Knowles accepts the criticism and says the teenager and his family were let down by a poor process. The officer is continuing to work with police.
Thursday's report is the latest formal criticism of police in Canterbury after a series of court cases and complaints.
Three officers recently appeared in court on charges ranging from bribery and corruption to theft as a servant.
The authority also upheld a complaint of excessive force used against a 67-year-old retired man and ordered compensation to be paid.