Environmental regulators are investigating an overflow from a dam at the Bengalla coal mine, making it three recent mine spills in the Hunter under review.

The Bengalla coal mine dam began to spill last Friday but there was no failure of the dam wall, according to mine operators, Rio Tinto.

Section of Rio Tinto's Warkworth sediment dam that gave way during the early January big rains. Credit:John Krey, via Newcastle Herald

This month walls at two other sites - Rio's Warkworth mine and Peabody Energy's Wambo mine - collapsed during heavy rains in the region, potentially spilling millions of litres of sediment-laden water into nearby rivers.

As late as Friday, water from the dam was still leaving the Warkworth mine site, the NSW Environment Protection Authority said in a statement. A spokesman for Rio, however, said the release of water from the dam had ceased on Friday.