The Independent Police Conduct Authority (IPCA) found several deficiencies in the original police investigations into alleged offending involving the Roast Busters.
IPCA chair Judge David Carruthers said police failed the group's alleged victims due to multiple "deficiencies" and failed to adhere to "basic tenets" of criminal investigation.
Following today's apology, sexual violence campaigner Louise Nichols said she was pleased police seemed to have learned a lesson from the case, and another said the case highlighted the need for a change in police culture.
Today's report comes after the IPCA received complaints about police investigations in November 2013 into the group - made up of predominantly West Auckland youths accused of bragging on social media about having sex with drunk and underage girls.
The IPCA report looked at seven cases in particular.
The report blasted a police failure to pursue positive lines of inquiry, and mentioned a lack of record-keeping and assessment of evidence.
It said at one point, "opportunities or interventions to prevent re-victimisation" were not considered. Officers failed to identify connections between incidents, or work with other agencies to develop plans to reduce the chance of a recurrence.
"Victims were let down by their failure to do so," Judge Carruthers said.
The report said although investigating staff treated young women with courtesy and compassion, investigations into original cases were not "robust and thorough".
Police did not check whether young men were involved in previous incidents and contact with the young men and their families was found to be "inadequate or non-existent".
Despite failings in the case, there was no evidence of poor practice nationally in child abuse or sexual assault cases, the judge said.
Louise Nicholas, who is involved in training crime managers after police realised that they had not "connected the dots" between complaints from Roast Busters' victims, said police had learnt the right lessons from this case.
"Hand on heart, I believe it can't happen [again] because of the systems that are now in place."
Since the investigation, Waitemata police had introduced several safeguards to ensure better "supervision and oversight of cases", as well as better liaison with Child, Youth and Family (CYF), to reduce the likelihood of the mistakes being repeated.
Waitemata district commander Superintendent Bill Searle and Police Commissioner Mike Bush both made public apologies after the report's findings were released.
"I'd like to publicly apologise to the young women and their families for the trauma that has been caused to them. I will arrange to do this in person, depending on their wishes," Mr Searle said. "We clearly let the victims down."
The standard of investigation in these cases "fell far short" of what was rightly expected, Mr Bush said.
He said he completely accepted the findings and police officers now need to do "whatever possible" to stop a repeat of the failures the IPCA identified.
"I'm very disappointed," Mr Bush added.
However, he said staff failures were isolated incidents. He confirmed Mr Searle would conduct a performance appraisal to determine where "accountability sits" in relation to police failures.
The IPCA recommended an audit of current cases the Waitemata child protection team was investigating be carried out, to determine if any "individual shortcomings" still remained.
It was also recommended police review what if any other practice or policy issues needed to be addressed, nationally and in Waitemata. Ensuring police were properly taught about laws relating to sexual offending was another recommendation.
Rape Prevention Education executive director Dr Kim McGregor said she supported a call for a taskforce to oversee and implement the IPCA's recommendations and change police culture.
She said there were "pockets of excellence" within police nationwide, but she'd like to see some police managers "weeded out", especially those she accused of having a poor attitude towards young women who reported cases of sexual violence, she said.
The Human Rights Commission also welcomed the Police apology.
"The Police apology is just the beginning, we have a lot of work to do. There are important lessons that the Police must take on board. They missed the opportunity to connect the dots, work collaboratively with other departments, confront the behaviour and prevent further harm to young women," said Human Rights Commissioner with responsibility for women's human rights, Dr Jackie Blue.
"Many sexual abuse victims have also been failed by the current judicial system. I support calls by Justice Minister Amy Adams for the Law Commission to re-start and prioritise this piece of work."
ROAST BUSTERS TIMELINE
* 2011: 15-year-old victim lays complaint with police about the group's alleged activities.
* 2011: Green Bay High School counsellor claims she went to senior management to raise concerns about alleged Roast Buster Beraiah Hales.
* November 21, 2013: 111,000-strong petition demanding stronger action over the scandal delivered to Parliament.
* April 2012: Green Bay High School receive allegations about Hales.
* May 2012: Hales leaves Green Bay High School.
* November 3, 2013: Videos emerge of a group of boys calling themselves the Roast Busters showing them laughing and bragging about having sex with drunk and underage girls.
Detective Inspector Bruce Scott said 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.
* November 5, 2013: Superintendent Bill Searle said none of the girls from the original inquiry wanted to make a formal complaint.
* November 6, 2013: A 15-year-old girl comes forward to say she laid a formal complaint with police in 2011 -- when she was 13. Police later confirm this is true.
* November 7, 2013: After calls for action, Police Minister Anne Tolley announces she has asked the Independent Police Conduct Authority to investigate.
* November 12, 2013: Police child sexual abuse specialist Detective Inspector Karyn Malthus appointed to head the newly named Operation Clover.
* November 16, 2013: Nationwide protests take place, with hundreds calling for an end to the country's 'rape culture'.
* November 17, 2013: A girl who says she made a complaint to police two years ago reveals she has made a second complaint after the Roast Busters scandal.
* November 21, 2013: A petition is presented to MPs outside Parliament with more than 110,000 signatures calling for more action for the alleged victims of the Roast Busters.
* December 12, 2013: Then Police Commissioner Peter Marshall grilled by a select committee in Wellington about the police response to the case.
* May 22, 2014: The Independent Police Authority releases its first report into the initial investigation, which focused on information provided by police to media, which said a "systemic breakdown in communication" by police led to inaccurate information being provided to the public, however, said "no individual could be criticised" for that.
* October 29, 2014: Police release the findings of Operation Clover and announce no prosecutions will be made.
* March 19, 2015: The Independent Police Authority releases a report criticising police handling of the initial investigation.
