As far as possible, they are subject to the same rules, routines and privileges as inmates in the general population.
A Corrections spokesperson said any rise in segregated numbers did not lead to increased costs.
"Voluntary segregated prisoners are housed in a unit which has the same facilities as a mainstream unit," the spokesman said.
"Any unit can be converted to a segregated unit at any time and the prison manager takes this into account when managing overall muster capacity."
Corrections said it could not explain reasons for the rise, saying that would have to be handled by another OIA request.
But top criminologist and Canterbury University professor, Greg Newbold, blamed the increase on the influence gangs had behind bars.
"Segregation numbers have increased a huge amount in the long term. It was less than 5 per cent [of the prison population] up to 1985," Newbold said.
"Now about 20 per cent of the prison population are in segregation."
Michael Murray, 34, was convicted on August 14 of the murder of Head Hunters associate Morris, 26, in West Auckland last year. Morris - the long-time partner of Millie Elder-Holmes - died after being struck on the head with a sickle.
Murray, who will be sentenced on September 11, was in segregation while on remand in the lead-up to the trial. It is understood he will be in segregation while serving his sentence.
Meanwhile, Corrections this week extended its investigation into Serco's management of Mt Eden Corrections Facility, where inmates have been taking part in organised fights, filming while apparently using drugs and drinking alcohol and also using an officer's walkie-talkie.
The deadline for the investigation has been extended from the end of this month to late October because of the volume of incidents it needs to investigate, and will include control of prisoner violence and inmate access to mobile phones.
