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.

Concerns of activists heard: Coal miners will face less strict environmental management conditions. Credit:Anita Jones

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.