The Corrections Department has been told it is putting the lives of teenage prisoners at risk by keeping them locked up for 23 hours a day in their cells at Mt Eden prison.
Prison inspections carried out by the Ombudsman have revealed about 70 teenagers are being held among adult prisoners and are allowed out of their cells for only about an hour a day.
The Ombudsman warned Corrections last year it needed to make changes but since then conditions for the teenagers have worsened.
When the Ombudsman's prison inspector went to Mt Eden in April last year there were 18- and 19-year-old prisoners being held in cells for 19 hours a day.
Corrections was told that was too long and to do something about it, it was also told to set up a dedicated youth unit.
Since that April visit Corrections has closed Waikeria Prison's youth unit and the young prisoners on remand are now sent to Mt Eden.
The Ombudsman's annual report found Corrections had not set up a youth unit and teenagers were still locked up with adult inmates with little or no access to programmes or activities.
In June there were 53 prisoners aged 18 and 19 at the prison and a further 18 younger than 18.
The Ombudsman said they are now locked in their cells for even longer and are allowed out for just one or two hours a day.
The annual report said young people in detention were extremely vulnerable and social isolation can be extremely distressing and even fatal.
A prison reform group said keeping teenage prisoners in cells for 23 hours a day was horrifying.
Jolyon White - president of the Howard League in Canterbury - told Morning Report the report made tragic reading.
"Young people shouldn't be there, and if there are going to be there, they need to follow the ombudsman's recommendation that they need a special dedicated youth unit.
"It's really not rocket science, dedicated youth unites are the way to deal with this. That, or to have a separate facility"
He said locking away teenagers with issues for almost the entire day was just making the problem worse.
"If they're angry going in, they're sure going to be angry after 23 hours locked away."
Mr White said the same issues of youths being locked away had been raised time and time again, but the problem had only got worse.
'A completely unacceptable approach'
Green Party corrections spokesperson David Clendon said locking children in cells for 23 hours a day in an adult prison was akin to torture.
"It is normally only the most violent, the most dangerous prisoners who are kept in their cells for 23 out of 24 hours," Mr Clendon said.
"Clearly, this is Corrections very poor attempt to make these young people safe by locking them away from other offenders, that's a completely unacceptable approach."
Kelvin Davis, Labour Party spokesman, said he was worried the conditions will leave the young remand prisoners feeling their lives were worthless.
"Their time out of their cells has been reduced from some five hours a day down to one to two hours a day," Mr Davis said.
"The big thing is when these young people emerge from prison, are they going to be in a better state of mind to assimilate and move back into the community or are they going to be in a worse position?
"Are they going to be more damaged and therefore probably likely to commit more harm and crime out in our communities?"
New Zealand First's youth affairs spokesman Darroch Ball said holding 16 and 17 year olds in an adult prison is a recipe for disaster.
"You talk about their mental health as well, they're having to keep them locked up for 23 hours a day.
"That's out of convenience for the Corrections Department, it's out of convenience for youth justice, it's not for the benefit of those youth.
"The Minister of Corrections clearly must know this is occurring, the Minister of Social Development clearly must know that this is occurring - they need be held to account.
"They need be questioned about why this is happening and what they're going to do about it."
Corrections Minister Sam Lotu-Iiga has declined to comment on the matter and Corrections would not make anyone available for an interview.
In a statement Corrections said it was strengthening the opportunities for youth at Mt Eden.
"We are actively working to increase the opportunity for young prisoners to participate in constructive activity while maintaining their safety, and the security of the site."
Corrections said it was also looking to appoint an officer young people could to talk to about any concerns they have for their safety.