A special "prison within a prison" is guarding the Christchurch mosque shooter and two other notoriously violent criminals at a huge cost to the taxpayer.

The facility, known as the Prisoners of Extreme Risk Unit, was set up four months after Brenton Tarrant murdered 51 worshippers and injured 40 others at the Al Noor and Linwood mosques.

Based within Auckland Prison but run separately, the unit is the operational and custodial function of the Prisoners of Extreme Risk Directorate – a group also established in response to the March 15 terror attack.

Its role has since been expanded and Corrections National Commissioner Rachel Leota says it now manages other inmates who present "an ongoing risk of serious violence".

It also supervises prisoners who have the ability to "influence others to engage in serious violence or threats".

As well as "violent extremists", Leota says offenders connected to organised crime groups may also fall under the group's purview given their "capability to seriously threaten the safety and security of a prison".

"It is a separate entity - a prison within prison," a source said of the Prisoners of Extreme Risk Unit.

"It's a secret squirrel operation. There is a special vetting system for staff to work there."

As well as the Christchurch gunman, the Herald understands two of New Zealand's most dangerous criminals, Hemi Te Poono and Siuaki Lisiate, are being held within the unit.

Corrections confirmed in response to an Official Information Act request three inmates were being held by the unit but would not elaborate further.

Leota also refused to release the number of frontline staff guarding the trio, although Corrections did confirm the unit "is managed by a team of carefully selected, experienced staff".

"Tarrant is in his own wing and there are 18 guards rostered to monitor him," a source told the Herald. "The other two are in the same wing but they are all dealt with individually. It's a costly exercise."

Corrections says the unit cost $2.77 million in the year to October 31, excluding the salaries of the six staff in its management group.

That compares to Corrections spending about $1.1 billion in 2020 to guard close to 10,000 prisoners across all its facilities.

The average population at Auckland Prison on any given day in the 2019/20 financial year was 522, a Corrections spokesman said.

A further $150,000 has been spent so far on modifications within the unit to protect the "health, safety and security" of staff and inmates. Leota refused to release specific details because it was "operationally sensitive".

The Herald's inside source said guards had little or no contact with Tarrant and monitored him on camera.

"The officers see him but they don't have a conversation with him. He reads and watches the officers watching him - that's all they do. They watch him on their monitors and take notes," the source said.

Corrections would not provide information on how many visitors (including family) Tarrant has had and who has been writing to him or how many times he has seen a psychiatrist.

"Like all people in prison, this individual receives three meals per day at times determined by the unit's schedule. Additionally, he is able to access television for a limited number of hours daily, has approved books to read if he chooses, and has access to his exercise yard twice daily," Leota said.

The Herald's source suggested Tarrant would eventually be sent to Unit 10, a maximum-security section at Auckland.

Unit 10 was the same part of prison where Tarrant's fellow inmate at the Prisoners of Extreme Risk Unit, Hemi Te Poono, is accused of kicking down a cell door.

"If Te Poono can allegedly kick down the door, how secure is it?" the source asked.

Corrections would not confirm any planned move for Tarrant but said Unit 10's cell doors were designed to withstand a considerable level of damage.

In August 2015, the 33-year-old Te Poono was sentenced to eight years and six months' jail for attempting to burgle the Manawatu Standard office in Palmerston North and a neighbouring building armed with a shotgun.

Last October he was charged with intentionally damaging a cell door and assaulting two prison officers, including a woman.

The Herald understands convicted killer Siuaki Lisiate is also in the unit. He was sentenced to preventive detention for stabbing murderer Graeme Burton, an amputee, more than 40 times with a shank. Burton is serving a life sentence with a minimum non-parole period of 26 years for shooting Karl Kuchenbecker dead in Lower Hutt in 2007.

Lisiate, a Crips gang boss, also known as JFK or Just F***ing Krazy ordered the 2009 execution of rival Bloods gang member, Tue Faavae, at Auckland Prison - the same place where Burton was attacked.

The source said Tarrant has had fewer death threats from fellow inmates after he pled guilty.

"But if the prisoners were constrained in any way, they would blame him. So many inmates would have loved the chance to attack him especially the 'heavies' in the gangs, they would want that reputation of 'nailing' him because he is high profile. The heavies would find a prospect and say , 'you do the deed and you'll get a patch.' "