Prison bosses have admitted that they expected riots when they banned smoking in jails last year.
The Corrections Department's principal research adviser, Dr Carolina Lukkien, told a criminology conference in Auckland that no country had banned smoking in all its prisons on a single day until New Zealand did it on July 1 last year.
"Smoking has long been part of prison culture. Two-thirds of prisoners are smokers," she said.
"It's fair to say we expected riots and expected things not to go as well as they did."
The ban made all tobacco products and lighters "contraband", imposing disciplinary consequences if prisoners or guards were found with them.
Dr Lukkien said the ban was effective. Confiscations plunged from 569 lighters and 237 tobacco items in the first month of the ban to two lighters and 12 tobacco items in June this year.
Nurses in all prisons offered nicotine-replacement patches and lozenges to all smokers for up to 12 weeks.
Dr Lukkien said some prisons held barbecues and concerts to involve prisoners in "celebrating" going smokefree, rather than seeing it as a hardship.
A second evaluation, completed this year by Wellington-based Litmus Ltd and Kaipuke Consultants, found that half of the prisoners who had been smokers said they either would not or might not start smoking again after leaving jail.
It said prisons provided more gym and other exercise facilities to help keep inmates' minds off smoking, and health staff reported health improvements as a result.
"Improvements in prisoners' self-esteem and confidence were also evident. Health staff reported prisoners telling them that having given up a nicotine addiction means that they feel they can give up other addictive behaviour also."
The evaluators visited five prisons and found tensions reduced after the ban at two but increased at two others.
"Across all prisons, there has been no observed increase in the number of reported violence incidents (assaults, standovers, intimidation, 'taxing' and fighting) leading up to or following the policy's implementation," the summary said.
But Corrections Association president Beven Hanlon, who works at Hawkes Bay Prison, said violent incidents had increased 180 per cent in the past two years, because of the smoking ban and increased double-bunking.
"It's a bit rich to say there were no riots," he said.
"At Hawkes Bay Prison we had two major incidents in the lead-up to the smoking ban. In one of them, prisoners barricaded themselves in a yard and demanded smokes and eventually got on to the roof.
"Corrections say those don't count because they were before the official start date," Mr Hanlon said. "But a month out, prisoners were no longer allowed to buy tobacco. That was the time we said there would be the most tension."