The EPA has issued one $15,000 penalty notice to Mount Thorley Operations Pty Limited for discharging saline and sediment laden water from an unlicenced point on January 5, 2021.

During a period of heavy rainfall, untreated mine water was unlawfully discharged into natural drainage lines leading to Longford Creek, a tributary of Wollombi Brook and the Hunter River.

A second penalty notice for $15,000 has been issued to Warkworth Mining Limited for a similar unlicenced discharge on January 4, 2021. This discharge also happened during heavy rainfall when untreated mine water was released offsite into natural drainage lines leading to Loders Creek, a tributary of the Hunter River.

EPA Director Regulatory Operations Adam Gilligan said both incidents had the potential to cause environmental harm.

"Sampling conducted by Mount Thorley Warkworth at the time showed elevated levels of pH in the water. It also showed increased salinity and sediment concentration. This can cause environmental harm to the surrounding creeks and rivers in the local community, including the Hunter River."

"These fines should serve as a reminder to industry that environmental incidents such as this can come at a cost, and preventing such incidents makes good business sense."

Water issues at the mine have resulted in previous prosecutions. In 2017 the then owners of the mine Rio Tinto were hit with a $50,000 fine over the collapse of a dam wall at MTW in 2016.

The company was prosecuted by the EPA over the failure, which allowed hundreds of thousands of litres of sediment-land water to escape from the dam and into surrounding areas.

Up to four megalitres of water is estimated to have escaped from the dam on January 6, 2016, some of it flowing into the Wallaby Scrub Road reserve.