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When the police opened the investigation last month they said they had received a small number of reports, but confirmed yesterday the number was now up to about 20 separate incidents.
Allegations made online say the musicians targeted women, men and minors.
The allegations include rape, sexual assault and violence. Police said enquiries were continuing, and no charges had been laid yet.
One man, who RNZ has agreed not to name, challenged one of the group members after a friend of his was threatened.
He told RNZ reporter Hamish Cardwell that since the allegations were first circulated online a number of survivors had told him awful stories about their experiences.
"It's heartbreaking that there's been that many men and women affected by these guys and their disgusting behaviour.
"It's heartbreaking and it's heartwarming, the fact that with the help of the community and the public and the ways they came together to combat this issue and speak up about this, it's given so many women, and men, the courage to come forth and get the justice that they deserve - and make sure that these guys are punished for what they've done.
"Because it's quite frankly, it's f*** disgusting.
"And it's quite powerful to know that these women have been brave enough to come forward and speak on their personal accounts because I can't imagine that's an easy thing to do at all.
The man said victims have told him "heart-wrenching" accounts of what had happened to them and that people had been afraid to go out at night downtown after being sexually assaulted by one of the men.
"You shouldn't have to be scared, you know, you should have the right to be able to go out and have a good time and not be worried that some creepy dude is going to try and drug you and take you home."
He said there were people out who had been attacked by the group who were yet to come forward.
"And I just want to encourage all of you to - I know ... it must not be easy, what's happened to you is truly heartbreaking and I stand with you.
"I just want to encourage anyone else who was affected to just come forward and speak."
Police said anyone with information should contact them.
Where to get help
• If it is an emergency and you feel that you or someone else is at risk, call 111.
• If you've ever experienced sexual assault or abuse and need to talk to someone call the confidential crisis helpline Safe to Talk on: 0800 044 334 or text 4334.
• Alternatively contact your local police station.
• If you have been abused, remember it's not your fault.