Double-bunking fails to deliver the type of prison the Government wants to see, a paper Corrections Minister Kelvin Davis presented to Cabinet shows.

But despite advice saying double-bunking fails to help prisoners rehabilitate, Mr Davis signed off on plans that rely on the practice.

Double-bunked cells have two beds in them, increasing the overall number of prisoners that can be held in a prison.

Corrections Minister Kelvin Davis provided the paper to Cabinet Ministers before the senior ministers signed off on plans to build a 500-bed high-security prison at Waikeria.

Half the new cells at Waikeria will be double-bunked, which means two-thirds of prisoners will be sharing a cell.

The double-bunked cells will measure nine metres-squared, even though the European Committee for Prevention of Torture states shared cells should measure at least 10 metres squared.

The paper says further double-bunking:

Reduces the opportunity to rehabilitate

Can negatively impact prisoner health

Creates a life different to one that may be led outside prison

Does not allow self-sufficiency

Three-thousand new cells would need to be built if we chose to "rapidly phase out" double-bunking, the documents say.

"The cells are purpose-built for double-bunking in that they are bigger, and so we think that will maintain prisoners' safety and staff safety," Mr Davis said in June.

"It's more budgetary. We don't want to double-bunk. We don't want to build prisons in the first place, but you know, that's where we've landed."

Other additional beds planned by the Government - 960 rapid build modular units and 422 emergency places - will also be double-bunked.

The paper, though, says the mental health unit is likely to "significantly benefit prisoners who suffer from high rates of poor mental health."

The prison population is expected to exceed the maximum safe capacity by 2877 places in 2021.

Newshub.