Corrections will not confirm whether John Vogel will receive compensation but continue to defend that the decision for him to be in solitary confinement was his own.
The Corrections Department acknowledged convicted murderer Vogel was kept in solitary for longer than the statutory maximum period, but that he had requested it.
Vogel was serving a life sentence for murder in 2000 when he pleaded guilty to three prison disciplinary charges relating to his drug use, a Corrections spokesperson said.
The spokesperson told NZ Newswire that Vogel appeared before a Visiting Justice for sentencing, and the prisoner himself requested a period of cell confinement of 21 days to be imposed.
This was in excess of the statutory maximum of 15 days.
A Visiting Justice is a District Court Judge or other qualified person, independent of Corrections, who is appointed to conduct disciplinary hearings and appeals relating to offences against prison discipline.
The Corrections response comes after a United Nations committee said Vogel should get compensation for unlawful solitary confinement.
During his solitary confinement Vogel was kept in a cell for 23 hours a day, had no access to a television or radio and was not allowed to make any telephone calls or receive visitors.
He had asked to be put in confinement to help him deal with his drug addiction but stated the conditions made him feel vulnerable, and argued he was deprived of "liberty and humanity" under the New Zealand Bill of Rights Act.
Vogel's lawyer, Peter Ellis, lodged a complaint with the United Nations Committee Against Torture in 2015 and last month received an advanced copy of their decision that said the confinement contravened international human rights.
The Corrections spokesperson said the UN committee's recommendation will be considered by the Government but made no mention of possible compensation.