The Department of Corrections must now recalculate the sentences of affected prisoners following the Supreme Court decision in Booth v R; Marino v Chief Executive of Department of Corrections [2016] NZSC 127, Corrections Chief Executive Ray Smith says.

Mr Smith says the Supreme Court ruling changes the way Corrections must calculate prisoner sentence dates, and in light of this it must recalculate sentences of affected prisoners.

He says 21 prisoners were identified who were immediately affected by the decision and they were released on 23 September 2016.

"Corrections has solid plans in place to ensure these releases are managed well from a public safety and reintegration perspective," he says.

"Around 500 serving prisoners out of the prisoner population of approximately 9,800 are also affected, and will have their release dates brought forward. In most cases this will only be by a matter of a few days or weeks rather than a significant period of time."