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.
"Samples of the toxic waste are being analysed, and officers have written to people in the area asking for information that might help track down the 'phantom dumper'.
"We want to work with whoever's responsible to help them dispose of this waste responsibly," Mr van Paassen said.
"But first of all we want it to stop."
The Western Treatment Plant treats an average of 3000 cubic metres of wastewater per day from Karori's approximately 5000 households.