Acting EPA director for the Hunter Lindsay Fulloon said the discharge occurred in November after debris from a mine blast blocked a sediment containment drain at the Liddell coal mine near Ravensworth.
"The EPA alleges that a combination of the blocked drain, a failure to inspect the blast area and heavy rain caused sediment laden runoff to breach the containment drain and flow into a pool in nearby Bowman's Creek," he said.
"We estimate 26,000 litres of sediment laden water, combined with around 170,000 litres of natural runoff from vegetation areas entered the waterway.
"Liddell Coal took action to pump the runoff back to the mine site and no environmental impact was identified."
Fulloon said there had been the potential for environmental harm, including from turbidity, which the EPA believed might have been avoided if the blocked drain had been cleared.
Liddell coal mine has developed a hazard plan to ensure similar incidents do not occur in future.
