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Fire Service staff responded at 8.35am, closing parts of three streets near the prison, in Liffey St, central Invercargill, for about an hour and setting up a hazardous chemicals decontamination tent on the driveway between the administration block and the prison building.
Police, St John and fire crews also attended.
Three administration staff, including the person who had opened the envelope, were put through a decontamination process as a routine precaution, Acting Senior Sergeant Deon McNaught, of Invercargill, said.
''There was no immediate or potential danger to any other staff, members of the public or prisoners, and the administration block was soon cleared and staff were able to return to work.''
The powder would be analysed to determine what it was, he said.
It was the second such incident in Southland in a week.
Last Wednesday, a Gore man found a mystery powder in his can of soft drink. There have also been several ''white powder'' cases reported in the North Island this month.