Auckland's Paremoremo prison has been flooded with sewage twice in two days.
Sewage flooded the prison's recreation room, an office, class rooms, and two dozen cells today.
Prison Director Tom Sherlock said it followed similar flooding on Monday.
There was a blockage in the system and staff were trying to identify the cause and prevent it from happening again, he said.
Those involved in the clean up have been given protective equipment, and all of the areas that were flooded have been decontaminated.