Christchurch City Council is investigating the death of more than 600 birds found along the banks of the oxidation ponds at the city's wastewater treatment plant.
It is working with National Institute of Water & Atmospheric Research and Fish & Game to determine how they died.
Dead birds have also been found in other wetlands in the east of the city.
The council says initial diagnosis indicates the birds died from avian botulism, a natural phenomena.
