Excess supply has again forced Vista Gold to discharge untreated waste water from the Mt Todd mine into the Edith River in the Northern Territory's north.
ABC News reports the US company made the discharge because a holding pond on site was close to capacity, and the water was tainted by heavy metals including cadmium and copper.
Vista says the release of two million litres of water was done in accordance with the mine licence conditions, and without the discharge the mine risked making an uncontrolled release during heavy rains.
Earlier this year Vista recorded an uncontrolled discharge from the Mt Todd site, and Government officials said heavy rainfall may have contributed to the release.
Prior to the release Vista was given permission to pump over 14gL of waste water from retaining ponds into the Edith River in order to reopen the mine.
Vista's operation is not the first mining project to adversely impact the Edith River, with an Oz Minerals train derailment last year spilling copper concentrate into the system