Corrections have confirmed they will double bunk at Arohata Upper Prison, a move criticised by the Ombudsman.

Chief Ombudsman Peter Boshier has released a report saying proposed double bunking - where there are two prisoners to a room - at the satellite prison inside Rimutaka Prison is concerning.

"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."

He said overall, the facilities and conditions at Arohata Upper Prison put prisoners at a severe disadvantage.

"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."

The Upper Prison's capacity is for 112 low/medium to minimum security prisoners, and the prison was full at the time of the inspection in late 2017.

Corrections, in response to the report, says a $10 million investment at the Upper Prison will give prisoners more time outside their cells; give them "constructive" activities including painting, decorating and tiling training; add two new yards; refurbish an administrative building as a programmes hub and upgrade gym facilities.

"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 per cent rise in the women's prisoner population in the last two years," says chief custodial officer Neil Beales.

He said the original plan to double bunk 88 of the 112 beds has been reduced to 44 now.

A new wing is also planned at the Arohata Prison in Tawa.