New Zealand's most violent prisoners are being transported on public flights in a move described as highly dangerous by prison staff and their union.

Inmates from Rimutaka Prison must be transferred to New Zealand's only specialist maximum-security prison, Paremoremo, within 48 hours of being reclassified as maximum security.

Previously the transfer has been made via road from Wellington to Auckland or by charter plane, but senior Corrections sources have confirmed prisoners began being transferred on Air New Zealand flights just before Christmas.

In response, the department said it had been shifting maximum-security prisoners via scheduled domestic flights "for decades" and there had been no policy change, despite the claims.

In 2007, then Corrections Minister Damien O'Connor, in response to safety concerns raised by the National Party, was reported as saying that only low-security prisoners were transported on commercial flights.

Each prisoner has been escorted during the trips by three uniformed guards.

At least four are understood to have been flown on Air NZ flights in the past month, with Arthur Briggs, who with four other inmates viciously attacked a guard in 2012, one of the first to be transferred.

Earlier this week another two prisoners were flown to Auckland, escorted by six guards. One of the prisoners was Dean Vincent, whose jail sentence for burglary was extended last year after he stabbed a fellow inmate in the neck with a knife made from a toothbrush and a piece of metal.

Another prisoner, who was reclassified after assaulting a staff member, was flown yesterday.

It is understood prisoners are handcuffed during the trip, but some pilots request that the restraints be removed during flight.

The move has bewildered staff, who believe it is dangerous to have such prisoners around the public, even if they are restrained, Fairfax Media has learnt.

"Maxi prisoners are the worst of the worst. Can you imagine the danger this is putting staff and the general public at?" one source said.

"These are guys who need three staff to unlock them and you are going to put them on a plane with the general public? Come on, it's a serious risk."

Corrections Association president Beven Hanlon said he had been shocked when told about the flights and in his 16 years as an officer and decade as head of the union had never heard of a maximum security prisoner being put on a plane with the public.

There were instances when normal and high-security prisoners were taken on scheduled flights, but maximum-security prisoners were deemed extremely dangerous.

"These are nasty people, they're bad news, but in New Zealand . . . we'll just chuck them on the plane at Christmas time with families and kids."

Department acting national commissioner Jeanette Burns said maximum-security prisoners had been flown on rare occasions on scheduled domestic airline flights for decades.

There had been no recent change in policy regarding their transport.

Each transfer was assessed on the safety and security of the public and staff, as well as the prisoner's needs such as health concerns.

Air NZ spokeswoman Kelly Kilgour said it was inappropriate to comment on specific security but there had been no change to the arrangement with the department.