"In investigating the police decision not to prosecute, the Authority found that the decision not to interview the officer as part of a criminal investigation was a departure from the proper process," he said.
"The authority also found that the decision not to prosecute the off-duty officer because it was not in the public interest to do so was made without proper consideration of the prosecution guidelines."
The incident was sparked at around midnight on June 12, 2011 when the off-duty officer heard someone smashing his letterbox outside his Fendalton home.
He went outside and saw a group of people in his driveway.
The officer went back inside, got dressed, collected his police baton, and went back outside.
By this time, the group had left the driveway but the officer could see a group of people nearby, walking away.
The off-duty officer got into his car and after catching up with the group he approached them on foot asking who had smashed his letterbox.
When one of the youths, a 17-year-old, told the off-duty officer he didn't know who was responsible the off-duty officer hit him multiple times with his baton, the IPCA said.
Later that day, the 17-year-old's mother contacted police to report the attack.
During the police investigation, the teenager and his parents were interviewed.
Despite wanting to minimise any impact it would have on their son's schooling, the 17-year-old's parents were insistent that some action be taken against the officer.
In August 2011, the Professional Standards Manager, who was also the Employment Practices Manager for Christchurch Police, submitted a report to the Acting District Commander recommending that the officer should not be prosecuted as it would not be in the public interest.
He considered the Solicitor-General's prosecution guidelines in reaching his decision.
The Acting District Commander approved a recommendation that it would not be in the public interest to prosecute the officer.
But today, the authority has found that the processes followed by Christchurch Police in reaching their decision not to prosecute the off-duty officer involved were unfair.
It further concluded that the decision suggests that the Professional Standards Manager had "prematurely decided" that a code of conduct investigation would be a sufficient resolution.
The IPCA concluded that any reconsideration of the prosecution decision at this late stage would be "an abuse of process given that the officer was informed he would not be charged prior to engaging in the employment process".
Canterbury District Commander Superintendent Gary Knowles said he would be offering to meet the victim and his family to apologise for the way the complaint was handled.
"There is absolutely no question that the actions taken by the officer against the 17-year old victim were unacceptable. I also accept that in this case the young man and his family have been let down by a poor process," he said.
"We have taken careful note of the issues raised by the IPCA and have made sure these are reflected in improvements to our processes for dealing with complaints.
"We have already made changes to the way we handle investigations into officers as a result of this incident.
Mr Knowles said the staff member who undertook the internal investigation had been spoken to.
"I believe the investigator and the then acting district commander acted with the best of intentions to ensure the matter was investigated appropriately.
"I am satisfied there was never any deliberate attempt to protect the officer or to minimise the seriousness of the offending.
"However the investigator made a judgement call to pursue an internal investigation rather a criminal prosecution, when a criminal investigation would have been the correct call.
Mr Knowles said the officer at the centre of the allegations admitted striking the victim and had been subject to disciplinary action, but he remained a serving officer.
