She said it was her first sexual experience and it left her traumatised.
She made a complaint to the police a few days later.
However, after two investigations into that complaint, and a raft of others relating to other teenage girls, police have not prosecuted the alleged offenders.
"This is just not good enough," said the mother, who cannot be identified for legal reasons. "Why? That is my big question? She was 13. Furious, that doesn't even begin to describe what I'm feeling."
Detective Inspector Karyn Malthus, who headed the investigation, called Operation Clover, said she accepted there was an expectation there would be charges.
"I would like nothing more than to be able to do that," she said. "But we are faced with having to operate within the rule of law."
The lack of admissible evidence and the refusal of some of the complainants to give statements to the police meant there was not a strong enough case against the alleged offenders to pass the test required under the Solicitor-General's prosecution guidelines.
Ms Malthus said the nature of the offence and the age of the parties at the time of the offending were also major factors.
The mother of the then 13-year-old cried as she spoke about what her daughter had been through.
"I feel like I have failed her, like I wasn't there to protect her.
"Before this she was a fun-loving, bolshie little toerag and she was on her way to see the world. Then after it happened, she went downhill."
She was bullied as a result of making her police complaint and had to leave two schools.
Her mother was constantly worried she would harm herself.
This year, she has pulled herself back from the brink and is now finishing NCEA with the hope of attending university.
"This will always be with her, though. It's her survival mechanism to block it out.
"She says she wants to move on, she's over it, she wants to get on with her life.
"I know that she will never get over it. It will keep coming up through her life and she will have to keep facing it."
Until she got justice and closure for her daughter, she would not rest.
A source close to the investigation said underage sex was one of the offences police considered during Operation Clover.
However, if a new complaint was made by the victim's mother, police would consider it.
Police said any new disclosures made to them regarding the Roast Busters would be investigated appropriately. There was no time limit for reporting sexual offending and they encouraged any person involved who had not yet come forward to contact them.
Sex abuse survivor and victims advocate Louise Nicholas echoed police.
"What information they have could be the missing piece police need to get the case to the prosecution stage."
Rape Prevention Education director Dr Kim McGregor said the odds were stacked against rape victims when it came to prosecutions.
She said getting a rape complaint past the prosecution guidelines was often too hard.
"I don't think the public realises how brutal and hostile the criminal justice system is on rape survivors."
Meanwhile, the family of Roast Buster Beraiah Hales were yesterday relieved the police investigation is over and say he has been vindicated.
Speaking for the first time, his older sister told the Herald that Beraiah's youth had been taken from him and while he had said and done some stupid things, he was no criminal.
"It has affected him in many ways. Mainly the loss of being able to live like an 18-year-old," said his sister, who didn't want her name used.
"It was hard on everyone in our family for different reasons, mainly because we know Beraiah's character. We know all young people do and say silly things ... "
She said a lot of time and energy had been wasted in investigating her brother.
"I am absolutely pleased about it being over ... He has done so well with the pressure, he has dealt with everything in a very mature manner and this has been incredibly tough on him mainly because he is innocent."
She said Mr Hales spoke to her about his involvement with the initial complainant, but she would not be drawn on it.
"I'm not saying what happened is right. It most definitely isn't, but the nation reacted in a way which has ended the youth of two very young men - only to have [an outcome] like this."
According to his Facebook page Mr Hales is in a new relationship, studying and living back in Auckland after going into hiding last year when the scandal broke.
"He is very happy," his sister said, "doing his own thing in his own way with the love and support of all of his family and true friends."
She was not angry with the police, but "disgusted" by the way the public reacted to yesterday's announcement.
Mr Hales was getting death threats sent to him through social media "worse than last time", she said.
"It's hard to say if it's anger I feel. The relief of hearing the positive result is more or less taken away from everything else.
"I'm not angry with the police," she said. "They have done their jobs and proving his innocence has proved that to me."
Mr Hales' sister said she found it "extremely hard" to see her brother's image on television and in newspapers during the investigation.
"That was actually the hardest part. Walking into shops and seeing the newspapers, that was by far the hardest part," she said. "As a family you have to deal with it in the most appropriate way you know how but to add to it with all the other stuff surrounding you is just very draining."
She said Mr Hales was ready to move on with his life, as were his family.
Mr Hales did not respond to messages yesterday.
But in January he told the Herald he was "not worried" about the investigation.
"As long as I know the truth I'll be fine," he said.
He described himself and co-accused Joseph Parker as "the most hated people in New Zealand" and refused to defend himself against "something that isn't true. As I said before, as long as I know the truth I'll be fine".
* Watch a video from yesterday's police press conference here.
How the case unfolded
2011: 15-year-old victim lays complaint with police about the Roast Busters 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.
April 2012: Green Bay High receives allegations about Hales.
May 2012: Hales leaves Green Bay High.
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 says although 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 says none of the girls from the original inquiry wanted to make a formal complaint.
November 6, 2013: 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 is 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: Girl who says she made a complaint to police two years before reveals she made a second complaint after the Roast Busters scandal.
November 21, 2013: Petition 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: Summary IPCA report on police response to media enquiries on Roast Busters released.
October 29, 2014: Police release findings of Operation Clover and announce that no prosecutions will be made because of insufficient evidence.
The three investigations:
* Operation Clover
The Waitemata Child Protection team's investigation into complaints of sexual assault made about members of the Roast Busters group. The initial complaint was made in late 2011 and no charges were laid because of a lack of evidence. The complaint was "reapproached" again in November 2013. The second phase of the investigation was dubbed Operation Clover and involved other agencies, including Child Youth and Family and ACC. At its peak the investigation team included a detective inspector, detective senior sergeant, detective sergeant, 10 detectives and specialist interviewers.
* IPCA #1
This investigation was into the botched handling of publicity around the Roast Busters case by police in November 2013. Police initially told the media that no complaints or formal statements had been received. That was incorrect. The IPCA found that a systemic breakdown in communication by police led to inaccurate information being provided to the public. However, it said no individual could be criticised for that.
* IPCA #2
An investigation into the adequacy of the police investigation into the initial complaint from the 13-year-old alleged victim and the reapproach. This investigation has been completed, and a report is in the final stages of being drafted. The final report is expected to be released before Christmas.
5 Facts about Operation Clover
The alleged victims
Of the 110 girls spoken to by police, 30 were believed to be the victims of some form of sexual offending, however, most refused to make formal statements.
The suspects
Seven girls made formal complaints to police about eight alleged incidents and five males were named as suspects from a list of 30 initial "persons of interest". Beraiah Hales and Joseph Parker were two of those suspects, who were all considered by police for prosecution.
Police process
Investigators seized and analysed computers, smart phones, social media activity and internet accounts. However due to a lack of admissible evidence none of the five suspects will be charged.
Alcohol a factor
Police say the prevalence of alcohol in the lives of the teenagers interviewed, both victims and suspects, was concerning. There was also a "poor understanding" of what consent was.
Future action possible
Police say that while no charges have been laid at this time, future prosecutions have not been ruled out. They said "there is no time limit for reporting sexual offending".
Who is Karyn Malthus?
In November 2013 police announced that one of the top female detectives in New Zealand would be taking over the investigation into the Roast Busters. In her role as Auckland City District criminal investigations manager Detective Inspector Karyn Malthus oversees all serious crime in the area. However with experience in dealing with sexual abuse cases, she was seconded to the Waitemata District to work on Operation Clover with the resident child protection team.
Ms Malthus joined the police in 1987 and has been working as a detective with the Criminal Investigation Branch for 21 years. Before taking the role at Auckland City she was the district crime manager for Northland and previously served in Counties Manukau and Waikato.
Among the high-profile cases Ms Malthus has spearheaded are the prosecution of Kaitaia deputy principal James Parker, who was jailed indefinitely for sexually abusing 20 boys under 16 over a 13-year period.
