Environmental regulators are demanding coal mining giant Peabody Energy explain how a dam wall at a Hunter Valley mine collapsed amid heavy rain and the company took a week to report the incident.
The miner told the NSW Environment Protection Authority on Monday its staff had only learned of the incident - which involved sediment-laden water breaching the dam - earlier that day, the agency said in a statement.
EPA officers visited the mine, near Warkworth, on Tuesday to carry out inspections, interview workers and take samples for testing.
It is understood workers could not reach the affected wall for several days because of safety reasons. The structure was a temporary holding dam and the sediment was mostly run-up and top soil rather than coal.
"Following recent heavy rainfall in the Hunter Valley there was an unplanned discharge from a sediment control structure dam at Peabody's Wambo mine complex," a spokeswoman for Peabody said in a statement.
During last week's heavy rains in the region, the Bureau of Meteorology issued flood warnings for the area, including the hamlet of Bulga, which abuts several coal mines.
EPA North Director Gary Davey said the agency is yet to determine how much of the mine's material left the dam site. The dam could hold as much as 3 million litres.
"Communities across the Hunter and Mid-North Coast regions have just experienced record rainfall over the last seven days," Mr Davey said. "While this is a factor to consider, the EPA's investigation will also examine how the dam, which was used as a sediment control measure, is managed, maintained and monitored."
The EPA said penalties could range as high as $1 million, depending on the severity of the incident. At this point, the water is considered to be mostly mixed with soil, sand, rocks and grass, with no known toxins.
Still, the dam failure has promoted calls for a clampdown on mines in sensitive regions.
"This shocking pollution incident highlights why the government should prohibit coal mining in good farming areas and in drinking water catchments," Jeremy Buckingham Greens NSW mining spokesperson, said.
"This massive spill follows recent pollution incidents at the Clarence colliery near Lithgow and Russell Vale coal mine near Wollongong and highlights the dangers of coal mining to both the natural environment and human health."
Farmers near the proposed Shenhua Watermark mine on the Liverpool Plains may face similar risks, he said.
Other incidents
Wambo mine was reportedly fined in 2012 and 2015 for incidents involving excessive blast fumes from explosives used in the open-cut operations.
It also has underground mining, with a combined total output of 7.2 million tonnes of saleable coal in 2014, according to the company's website.
The EPA will review the mine's monitoring systems to identify why the company took so long for the wall collapse to be reported.
"The EPA is also concerned that it took nearly a week for mine personnel to discover the problem and report it," Mr Davey said. "[i]n the event of a failure, it is vital that there is an early warning process so that action can be taken quickly and the EPA notified.
"The Hunter River catchment is a sensitive environmental area, relied upon by industry, farmers and the wider community for its freshness and good water quality."
