The Wellington City Council and a water company are tracking a "phantom dumper" who has been disposing potentially dangerous material into Karori's drainage system.
Toxic material was being discharged into the wastewater network in the suburb about every couple of weeks, often in the small hours of the morning, water
network company Capacity Infrastructure Services said.
The material was toxic enough to threaten the biological processes of the suburb's wastewater treatment plant, Capacity spokesman Alex van Paassen said.
"In a worst case scenario, a large load of this stuff could kill the treatment plant. That could lead to untreated sewage being discharged off the south Makara coast for however long it takes to get the plant up and running again."
It also threatened the health and safety of workers at the plant, Mr van Paassen said.
