A unit manager and three inmates at Rimutaka Prison have been charged with cannabis-related offences.
The arrests follow an investigation involving the Department of Corrections and police.
Police laid charges against the unit manager on Thursday and the prisoners were to face charges on Friday.
Department of Corrections chief executive Barry Matthews says his staff quickly detected the illegal activity and worked closely with police to identify the accused.
Mr Matthews says any kind of illegal activity cannot be tolerated in prisons and he expects a high level of honesty and integrity from staff.
He says the actions of one manager should not detract from the hard work, dedication and professionalism shown by staff at Rimutaka Prison.
The department will not comment further on the nature of the charges, saying the matter is now before the courts.
Mr Matthews says the arrested prison manager has worked for the Corrections Department for 16 years. He says indications are that no other staff from the prison were involved in the activity.