Despite threats from Mitchell the victim sought medical attention and police later became involved.
JustSpeak director Tania Sawicki Mead said it was not only sex offenders who posed a risk in a double-bunking scenario.
“What the evidence tells us is that it’s not a safe or appropriate place for anyone to be kept,” she said.
The recently released New Zealand Crime and Victims Survey showed the public believed the most important part of the criminal justice system was to help offenders stop reoffending, rather than punishing those who commit crimes or deterring them from future offending.
Ms Sawicki Mead said the status quo in prisons did not correspond with that attitude.
Prisons were full and double-bunking had become the norm.
She said that would not be altered until the Government made fundamental changes that would significantly reduce the rates of incarceration.
They would have to look at alternatives for those sentenced to short prison terms and/or the threshold for those seeking bail, she said.
“There’s a clear understanding the current system is failing everyone,” Ms Sawicki Mead said.