A union representing prison officers says a new law designed to reduce contraband in prisons won't work unless the Government employs more guards.
An amendment to the Corrections Act passed by 113 votes to nine on Tuesday night with the only Greens voting against.
Corrections Minister Judith Collins says the amendment will increase control over inmates' communication with the outside world.
The amendment allows for electronic detection and interception of cellphones. Officers will be able to screen prisoners' mail.
It also increases the penalties for anyone found on prison grounds with banned items.
The Corrections Association is delighted with the new law, but says there are not enough guards in New Zealand prisons to carry out the new tasks.
It says Australian prisons have teams dedicated to screening mail, testing for drugs or searching people for contraband.