About 2100 prisoners are being forced into early lockdowns at 5pm every day because of staff shortages in the Corrections Department.
Corrections assistant general manager of operations Bryan McMurray said the new policy could last well into next year and beyond, depending on the success of recruitment drives
or swings in the labour market.
Prison staff are bracing themselves to be further stretched over the Christmas period, as officers take annual leave.
Welfare groups are concerned the policy, which deprives prisoners of social activities, is having an impact on the chances of rehabilitation.
The lockdowns have been implemented at Rimutaka, Arohata Women's and Wellington prisons and Waikeria Prison in the Waikato. They have been in place at Waikeria for more than three years.
About 2100 prisoners have to have dinner at 4pm and are locked in their cells from 5pm to 8am every night without access to social activities, prison facilities, visitors or phone calls. A standard single cell is 8sq m.
Normal lockdown hours are generally 8pm to 8am, depending on a prisoner's security risk.
Lyanne Kerr, national director of NZ Prisoners Aid and Rehabilitation Society, said the lockdowns reinforced an anti-authority mentality. "A lot of prisoners think this is unfair. This is an operational issue and has nothing to do with them."
She praised department programmes such as literacy and drug-treatment courses, but said it was a "shame to be affecting the effectiveness of that, just because we can't get enough people to walk the floor".
Mr McMurray conceded the policy was not ideal and may even last into next year and beyond.
National Party corrections spokesman Simon Power said he was concerned the lockdowns would lead to an increase in complaints, which could lead to compensation claims and taxpayer-funded payouts.
But Mr McMurray said there had been neither an increase in complaints nor claims for compensation in the past year.
He said the Waikeria experience had even had a silver lining. "Many of them say they prefer the early lock-up. It's less opportunity for them to get into trouble."
