Chief Ombudsman Peter Boshier says double-bunking cells planned for Arohata Upper Prison fail the international standard for humane treatment, hurting prisoners' chances of turning their lives around.
Meanwhile Justice Minister Andrew Little says the lack of a mandatory comprehensive medical check for every prisoner means that their mental health needs are falling through the cracks, further denting chances of rehabilitation.
In a report released today, Boshier said that the 44 cells in Arohata marked for double-bunking "did not meet the international standard for even one person, let alone two".
The European Committee for the Prevention of Torture and Inhuman or Degrading Punishment defines the minimum space for a one-person cell as six square metres, plus sanitary facility.
The double-bunking cells in Arohata are 5.7sq m, including sanitary facility.
"All the women at Arohata Upper Prison are low security, yet they're managed as if they're high security, with much less time out of cell, little or no constructive activity, and no access to the programmes they need to complete for successful parole," Boshier said.
"Adding double-bunking to an already cramped and limited environment will have a very negative effect on the women's physical and mental wellbeing, and their prospects for rehabilitation."
The Government has a target to reduce the prison muster by 30 per cent in 15 years, and an announcement on whether to go ahead with a $1 billion prison facility in Waikeria is expected in about two weeks.
Little, who is in Europe for a number of justice-related meetings, said Dutch prisons were at 75 per cent capacity, while New Zealand prisons were at 93 per cent capacity.
He also met with officials at the Scottish Criminal Cases Review Commission, who "fell off their chairs" when he told them the size of New Zealand's prison muster.
The latest Corrections Department figure for the prison muster is 10,470, for the end of last September.
Little visited a prison in Dordrecht in the Netherlands, where he said recidivism rates had been cut by 10 to 15 per cent in the last decade.
"Every new prisoner that comes in is given a medical examination and a health screen, so they learn about health and mental health conditions and other issues.
"In New Zealand, if a prisoner volunteers a condition that needs medication, then prison authorities will know, or if their behaviour indicates there are issues, then prison authorities might respond.
"But there is no process of a full medical screen ... I would have thought it was a pretty obvious thing to do in order to manage prisoners effectively."
He would like to see a full medical check for every New Zealand prisoner, but would need to take proposals to Cabinet.
Dutch prisoners also had a folder containing all the programmes and work they did while in prison, mapping their progress.
"They do less time locking up and more contact time, working in workshops or cleaning or something productive.
"We went to their workshops. Just talking to me and their demeanour towards each other, it wasn't the heavy atmosphere of New Zealand prisons I've visited."
Little said there were still a number of options to canvass before making a decision on Waikeria.
Prime Minister Jacinda Ardern has said that she wanted to visit Waikeria before deciding if the prison needed replacing.