The Corrections Department rejects suggestions that cell lockdown periods have been extended because of staffing shortages.
The Corrections Association says prisons have been operating under extended lockdown periods for the past 18 months.
The Howard League for Penal Reform's Canterbury secretary, Kathy Dunstall, says two units at Paparoa and Waikeria prisons are being locked down from 2.30pm - 8am, much longer than before.
But the department says staffing levels are not a problem and lockdown hours have not been extended.
The department says it makes changes to its lockdown hours close to Christmas, so prison officers can take leave.
