Mine dam wall collapse investigated

Environmental authorities are investigating the collapse of a NSW Hunter Valley coal mine dam that went undetected for up to six days.

Part of a dam wall at the Wambo mine in the coal-and-agriculture-rich Hunter region has collapsed, releasing sediment-laden rainwater into the environment, the NSW Environment Protection Authority (EPA) said on Wednesday.

EPA North director Gary Davey told AAP the amount of water released was not known but the dam - built to hold rainwater runoff - could hold up to three million litres.

Wambo is owned by US energy giant Peabody.

The EPA was told of the dam collapse on Monday when it was noticed by mine staff but the company has told the regulator the failure may have occurred almost a week ago.

The NSW Hunter region was hit by extremely heavy rain in the past week, with flooding in many areas.

The EPA said while the heavy rainfall would be considered, management and monitoring systems at the mine would also be looked at as part of the investigation.

"The EPA is also concerned that it took nearly a week for mine personnel to discover the problem and report it," Mr Davey said.

Mr Davey said delays in reporting incidents such as a dam failure could be serious depending on the materials involved.

He said the failed Wambo dam did not contain mine tailings or material from a coal washery.

With the EPA investigation in its early stages, Mr Davey said it was too soon to say whether any breach of the mine's environmental licence or relevant legislation had occurred.

In a statement, a Peabody Energy spokesperson said the dam in question was inaccessible during the recent heavy rain in the Hunter Valley and the EPA was notified as soon as mine staff could safely inspect the area.

The spokesperson said there had been "an unplanned discharge from a small temporary sediment control dam" that captures rainwater at the Wambo mine complex.