Environment Southland are encouraging people to remain cautious with the water after sulphuric acid was discharging into the Mataura River overnight (5 Sep), following a mechanical fault at the Gore District Council's (GDC) wastewater treatment plant.

GDC staff immediatedly shut down the plant on Tuesday morning (5 Sep) after discovering that the highly corrosive acid had been discharging into the river for up to 12 hours.

A GDC statement said acidity tests (pH) of the river, from near the wastewater plant to Mataura township, have shown no abnormal readings.

GDC staff are working with Environment Southland who are continuing to monitor water quality.

ES Acting General Manager - Integrated Catchment Management, Donna Ferguson said the Environment Southland compliance team was on site today (5 Sep) and took samples and readings on water probes which they would be analysing.

"Contamination of any kind to a waterway is of concern and could have impacts on plant and aquatic life."

"A cautious approach is always advised when interacting with waters that might have been impacted by contamination," Ferguson said.