The Government ban on prisoners smoking in jail could lead to increased riots and disorder, the Corrections Department has warned.

But it noted that a long implementation period that included quitting programmes would reduce the risks.

In June, Corrections Minister Judith Collins said prisons would be smoke-free by July 2011. Prisoners would not be able to smoke or carry tobacco or lighters, and prison staff would be allowed to smoke only in designated outdoor areas.

The Corrections Association criticised the move as putting staff in danger, but Ms Collins said a ban would be healthier and prevent legal action against the department for failing to provide a safe environment.

About two-thirds of prisoners - or 5700 - smoke. They can smoke in their cells and certain outdoor areas.

Department advice to Ms Collins - released to the Herald under the Official Information Act - was that smoking restrictions and bans overseas had not led to a general rise in incidents.

"However international experience shows that prisoner riots and disorder in prisons have occasionally been linked to smoking bans," the paper said. "These disturbances are usually exacerbated by poor implementation practices where prisoners and staff are unprepared for and/or unsupported during the period leading up to [the ban]."

The paper also noted that a ban in Wichita Falls Psychiatric Hospital, in the United States, had eventually led to a decrease in assaults because many conflicts had been over tobacco.

Corrections considered letting prisoners smoke outside in certain areas, but was worried this might lead to incidents that could draw staff to these areas away from other duties.

A total ban would be easier to police.