The failure of a dam wall, for which Whitehaven Coal failed to maintain an inspection schedule or a plan of maintenance, has landed the company a $15,000 fine.
The NSW Environment Protection Authority said the sediment dam, part of the company's Tarrawonga Coal Mine, failed under heavy rain in February.
The miner did not have a maintenance plan in place for the dam and lacked an inspection schedule which could have helped identify emerging structural failings, according to the EPA.
The company is revising its maintenance plans to ensure the sediment dam failure does not happen again.
Whitehaven was contacted for comment, but did not respond to the Leader before deadline.
Tarrawonga Coal Mine, which is about 16 kilometres north east of Boggabri, employs 180 workers around the clock.
It produces about 3 million tonnes of thermal coal and semi-soft coking coal for export every year and uses the dam to mitigate pollution, holding and treating sediment-laden water generated during mine operations.
EPA Manager of Regional Operations Lindsay Fulloon said the dam needs to be regularly maintained.
"A sediment dam wall breached during heavy rain in February this year, causing sediment-laden water to discharge onto a neighbouring coal mine," he said.
"Fortunately, the water was contained on this neighbouring site and did not make it to any waterways."
Lock the Gate spokesperson Georgina Woods said it proves the company is a "rogue operator" and "repeat offender".
"The company has been investigated or found in breach of environmental laws or conditions on more than 20 occasions since 2012, across five different coal mining sites in the Namoi Valley," she said.
"It's clear the many fines...have done nothing to discourage this irresponsible behaviour and that more severe penalties and enforcement is needed."