One of the complainants was just 13 at the time she was allegedly assaulted by at least two of the Roast Busters. Her mother blasted the IPCA and police when she was alerted to the report by the Weekend Herald.
The mother, who cannot be named to protect her daughter's privacy, said she was not informed of the IPCA report or the police apology before the release.
"I'm so f**king angry. What good is [an apology] now after four years?"
The report showed problems in police handling of the complaints.
The report said officers failed to pursue positive lines of inquiry, did not identify connections between incidents and "opportunities or interventions to prevent re-victimisation" were not considered.
Waitemata district commander Superintendent Bill Searle and Police Commissioner Mike Bush both admitted the standard of police work in the case had been unacceptable.
Social Development Minister Anne Tolley, who was Police Minister when the Roast Buster complaints were laid, has announced an investigation into CYF's involvement in the case.
Professor Mark Henaghan, dean of law at Otago University, said the youths could still be prosecuted.
But a police spokesman yesterday said there was nothing in the report which has not already been considered.
- Additional reporting, NZME.