Police have released the findings in the Operation Clover investigation into eight incidents, involving seven victims who made formal complaints, and five suspects.
A further 25 girls declined to make formal statements, but were believed to have been victims of sexual offending.
Detective Inspector Karyn Malthus, who headed Operation Clover, said there was not enough evidence to prosecute.
"Other factors included the wishes of individual victims, the admissible evidence available, the nature of the offence and the age of the parties at the time of the offending."
The overview of the investigation report, written by Ms Malthus, has been released to media today.
Detective Superintendent Andy Lovelock said this was released "to provide transparency and assist the public in understanding the complexities involved".
The report said an extensive analysis of social media identified 110 girls that police canvassed in relation to the case. Five of those girls provided formal statements.
In total, 35 males were considered by police in relation to eight incidents involving seven victims, including two girls who complained received prior to the launch of Operation Clover.
The main offences investigated were rape and unlawful sexual connection, and sexual conduct with a young person under 16.
"Throughout the investigation the priority was for the welfare and privacy of the girls involved, and ensuring that all support options were made available to them," Ms Malthus said.
"We have emphasised to both the victims and suspects that there is no time limit for reporting sexual offending."
Commissioner Mike Bush said in a statement that he was confident police conducted a thorough, and professional enquiry in to the case.
He described the case as "challenging and complex".
"I accept the decision not to lay charges will prompt a range of reactions.
"The behavior of this group caused a significant public response and there was a strong expectation in the minds of many that a prosecution would result.
He said he would await the findings of an Independent Police Conduct Authority into the initial handling of the investigation.
"We know that sexual assault in all age groups is under-reported. I am committed to ensuring that victims of all ages have trust in police and they can be assured their complaint will be thoroughly and professionally investigated."
Child Youth and Family worked with police during Operation Clover to ensure any young woman under 17 and their families had access to therapeutic or other support.
The release of the findings of Operation Clover comes after the IPCA found a systemic breakdown of communication within the police led to inaccurate information being provided to the public amid the Roast Busters scandal.
The IPCA report was released on May 22, and looked into the adequacy of the police investigation and the handling of any complaints or reports to police from the public between 2011 and October 2013. It also looked at the information police gave to media.
The Roast Busters scandal caused widespread outrage after it was revealed young men were posting videos of themselves online bragging about sexual activities with drunk, underage girls -- some as young as 13.
Detective Inspector Bruce Scott of Waitemata district said in November 2013 that even though police were aware of the group, there was nothing they could do until a girl was "brave enough" to make a formal complaint.
Days later it emerged a young teen had complained to police two years before, but she was not taken seriously.
The 15-year-old who went to police about the Roast Busters in 2011, said she would lay a new formal complaint after the scandal broke. She also criticised police for their line of questioning, asking her what she was wearing at the time of the assault, when she was aged just 13.
By the numbers
110 girls canvassed
66 girls eliminated following canvassing
44 girls re-approached for clarification
14 of the 44 girls eliminated upon re-approach
5 of the 44 girls made formal complaints
25 girls declined to provide formal statements, however are believed to have been victims.
2 girls made complaints prior to Operation Clover and re-engaged with police
7 girls in total made formal complaints
Persons of interest:
30 males identified as persons of interest
22 were interviewed
4 were unable to be identified
3 refused to be interviewed
1 was overseas
2 were under 17 at time of interview
28 were over 17
25 - the age of oldest person of interest
Suspects:
5 males named as suspects
3 agreed to interview
2 declined interview
4 of the suspects were 18 years old
1 of the suspects was 20 years old
- NZME
