The Independent Police Conduct Authority has found multiple failings by Southern District police in their investigation of a car accident caused by a local officer.
In July 2005, an unmarked police car driven by Senior Constable Neil Ford was involved in a crash with a car driven by Shane Cribb near Alexandra.
Mr Ford and Constable Dairne Cassidy were convicted of attempting to pervert the course of justice by trying to show that the accident had been caused by Mr Cribb, when in reality it was Mr Ford's fault.
The authority said that Alexandra police showed a lack of adequate supervision and did not address the concerns of Mr Cribb in a timely manner.
However, it said that while the investigation was mishandled, it did not believe there was a cover-up or conspiracy between the two officers. It said the situation is unlikely to be repeated, because a new code of conduct procedures has been introduced.
Shane Cribb's friend Stephen Potter, who took the case to the IPCA, told Radio New Zealand's Checkpoint programme on Friday there needs to be a lot more independence in investigations.
"Police investigating a police accident, police investigating fellow policemen - maybe friends or colleagues investigating each other. That sort of situation it's hard not to be truly unbiased in situations like that."
District Commander Superintendent Andrew Coster told Checkpoint that relationships with police were damaged and have had to be rebuilt.
"I acknowledge there is a perception around the way this case was handled. And really the only way for us to move forward with that is to demonstrate by our conduct from this point that we're able to operate with the independence and the integrity that the public reasonably expects."
Mr Coster said was a single incident and doubts the police would find another case as disappointing as what happened in 2005.