Human error has being blamed for the discharge of 450 litres of surphic acid, from the Gore District Council's (GDC) wastewater treatment plant, into the Mataura River yesterday (5 Sep).

The 12-hour discharge was first noticed by staff yesterday morning, prompting the plant to be immediately shut down.

GDC 3 Waters Operations Manager Aaron Green said the discharge was the result of a step being missed during maintenance at the plant.

"The Council takes its responsibilities seriously and will work with staff to ensure additional safety measures are implemented and retraining occurs," Green said.

Green said four 900 litre containers of sulphuric acid, which are used to correct pH levels, were stored at the wastewater treatment plant, with only one being connected at a time.

Southland Recreational Whitebaiters Association President, Roger McNaughton, said any discharge that toxic, was unacceptable in today's modern times.

"Any whitebaiter would be concerned."

"Surely there's safeguards that stop that sort of thing happening.... [and] what safeguards are in place to stop it [from] happening again," McNaughton said.

GDC reported no abnormal readings in the river after yesterdays discharge however an Environment Southland compliance team was directed to take samples and readings on water probes which were to be analysed.