Otago Corrections Facility staff got to the bottom of a drug smuggling attempt last week, after a prisoner tried to bring in drugs internally.
Prison manager Jack Harrison said staff became suspicious that a remand prisoner might have been concealing contraband.
After an interview with the on-call manager and staff, the man was placed in a dry cell, as prison staff are prohibited from making a body cavity search.
Except for a mattress, the dry cell was empty, having neither a washbasin nor a toilet.
"Those who are suspected of concealing contraband are placed in here to wait out the appearance of further evidence," he said.
Over the next two days, staff discovered several capsules in the man's bowel motions.
Mr Harrison declined to name the drug in the capsules, but said it was a prescription medicine used to treat ADHD.
The prisoner had been charged under the prison's internal disciplinary system and the police had been notified.
Mr Harrison praised his staff for their efforts in finding the contraband, which was "not a particularly pleasant job".
The dry cell was used about once a month when smuggling was suspected.
Most prohibited items were discovered in communal areas of the prison, he said.
