THE Environment Protection Authority is investigating after BHP Billiton reported water rushing from its Mount Arthur coalmine on to the Hunter River floodplain yesterday following torrential rain.
An unknown amount of water from the mine site flooded Denman Road for a short period after the downfall.
The water is believed to have accumulated behind a levee built close to the road.
Some houses in the area, believed to be owned by the mine, were sandbagged.
An EPA spokeswoman said the company had advised the water was runoff, and "as far as we know it is not water from the mining operation itself".
"We will certainly get more information and follow up on the report," the protection agency spokeswoman said.
The Newcastle Herald was told the water left the mine site through a gap in the levee that was built after the coalmine was approved to operate within 50 metres of Denman Road.
"It does appear there was a lot more water trying to get through the area than the mine had ever anticipated," a source told the Herald.
Mount Arthur coalmine is the Hunter's largest open-cut mine, producing 32 million tonnes of coal per year.
BHP Billiton has an application at the NSW Department of Planning to extend the mine for another four years, and increase train movements from 24 to 38 per day.
In a submission to the department as part of the mine assessment process, the Nature Conservation Council of NSW said the "greatest concern" about water management at Mount Arthur mine was "the potential impact of mining on the alluvial aquifer associated with the floodplain of the Hunter River".
The council criticised modelling data that "does not provide a strong foundation for assessing potential risks and impacts".
BHP Billiton was contacted for comment but had not responded last night.