Corrections Minister Kelvin Davis said smaller cells was one of the concessions with the budget he was given.

The new double-bunked cells at Waikeria Prison will not meet the international standard set out to prevent torture and inhuman or degrading punishment.

The guidelines for the minimum size of a two-person cell is 10 metres square of living space, plus sanitary annex.

But Corrections Minister Kelvin Davis has said the Government's plans for the new cells at Waikeria Prison do not meet that criteria, and will be only be nine metres square.

The Government announced it will build a new 500-bed facility at Waikeria to help relieve pressure on the bursting prison system - including a 100-bed mental health facility.

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The 600-bed prison was due to be completed by early 2022 and was expected to cost about $750 million.

SUPPLIED International standards for prison cells are set out by the European Committee for the Prevention of Torture and Inhuman or Degrading Punishment.

Half of all cells at the new facility will be double-bunked, meaning two-thirds of prisoners will share a cell - a practice Labour and the Greens have been critical of in the past.

"We are trying to to the best we can with the budget available," he told RNZ on Thursday morning.

"We had to make some concessions and that's one of them. I would prefer that people weren't in prison in the first place and that's what we are really trying to focus on."

JASON DORDAY/STUFF Mike Williams thinks the new mental health facility is a "very good idea".

The European Committee for the Prevention of Torture and Inhuman or Degrading Punishment (CPT) set out the international standards for a one-person cell at six metres square of living space, plus sanitary facility.

In a 2011 CPT report the CPT stated accommodation in double cells measuring between 8 and 10 metres was not without discomfort, and recommended avoiding placing two prisoners in 8-metre cells.

Prisons are not meant to be a resort, Davis said.

"We are confident that the prison we are building will meet the needs of our prison population, and the needs for staff safety.

"It's not the best situation, but we do have to keep the cost down to the taxpayer."

However, Chief executive for the Howard League for Penal Reform said saving on space was a justified compromise to modernise a prison that is "past its use by date".

"I think it's a good compromise, the top jail that is being replaced was built in 1905, it's very much past it use by date."

Replacing it with a bigger modern jail is sensible idea, he said

"The Howard League thinks the mental health facility is a very good idea."

"It doesn't make a lot difference to me that it will be slightly below the international guidelines, what is relevant is the opportunities to rehabilitate and improve yourselves."