Victims were let down by police failures in the investigations into the Roast Busters case, the Independent Police Conduct Authority (IPCA) says.
The IPCA investigation into the alleged offending by a group of 17 and 18-year-old youths, who boasted online about their sexual exploits with drunk and underage females, has found significant deficiencies in the original police investigations.
The IPCA's second report into the controversial case says the investigating officers tended to approach each case on an individual, case-by-case, basis instead of developing strategies to reduce the recurrence of what was "clearly unacceptable and, in some cases, criminal behaviour".
Chairman Sir David Carruthers said the Authority believes the officers should have identified the connections between the various cases and worked with other agencies.
"Victims were let down by their failure to do so," Sir David said.
Police began investigating the group in 2011 after girls as young as 13 complained they had been sexually violated, but no charges were ever laid because investigators believed there was no reasonable chance of a conviction.
Mr Carruthers said there were deficiencies in the investigation, including a failure to follow up and pursue positive lines of inquiry. He also said the supervisory oversight of the individual cases was inadequate.
"Investigating staff failed to properly consider all available offences in determining whether or not to prosecute the young men."
Sir David said all the officers involved treated the young women and their families with courtesy and compassion and maintained good contact with them but he said the officers' contact and interaction with the young men who were the subjects of the investigations were inadequate or non-existent.
"The failure of police to make contact meant the young men's parents were never made aware of several of the incidents and details of their sons' involvement and therefore they were unable to intervene or act to address the behaviour."
An earlier investigation by the IPCA criticised police over inaccurate information provided to the media on the case, and prompted an apology to the complainants.