The Chief Ombudsman says double bunking at Arohata Prison would break international torture and degradation rules.
Judge Peter Boshier has made 25 recommendations into his assessment of the upper prison. He said the 5.7-square-metre units don't meet international standards.
"Cell sizes at Arohata Upper Prison don't meet the international standard for even one person, let alone two," Mr Boshier said.
"The facility simply won't be suitable for double-bunking without significant improvements and additions, and even then double-bunking should only ever be used as a temporary, emergency measure."
"We can't just increasing the population in prisons unless there's somewhere civilised in which to put them."
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Corrections will now reduce the number of double bunks from 88 to 44, and spend $10 million on a facelift of the women's prison.
"Some of the challenges faced at Arohata are the direct result of the need for us to rapidly increase prison capacity in response to a 40 percent rise in the women's prisoner population in the last two years," chief custodial officer Neil Beales said.
"We have to take prisoners from the courts, we can't turn them away at the gates because we don't have any space, so we recognise that the Ombudsman was concerned about that, but equally where do we put the prisoners? We can't put female prisoners in a male prison."
There are also plans for a new wing at the Tawa prison.