The mine, which is 85% owned by Idemitsu and also has South Korea's LG as a shareholder, is one of Queensland’s biggest thermal coal mines. Its changing fortunes signal the pressure coal miners are facing to maintain margins in a climate of sinking thermal coal prices.

Rio Tinto Coal last month blamed low thermal coal prices for its decision to retrench employees from its Clermont open cut coal mine in Queensland less than two years after the $US1.29 billion operation was opened.

Prices for thermal coal, used in power generation, have slumped because of waning demand and increased exports from the US, where lower gas prices have prompted substitution.

The number of contract staff at the Ensham mine had been reduced by approximately 100, Ensham Resources chief executive Peter Westerhuis reportedly confirmed to The Australian.

The Idemitsu website says the mine has 600 employees and contractors.

"With the thermal coal price depressed, some areas of the mine are currently uncompetitive," Westerhuis reportedly said.

"With low prices and increasing input costs, the decision to bring forward the conclusion of a mining services contract has been required to maintain the business on a profitable footing.

"If the price stays as low as it is, we will have to review further parts of the business, but we haven't made any decisions yet."

Open cut coal mining is currently conducted by a large prestrip fleet and four draglines, the largest dragline being a Bucyrus 8750.

Once exposed, the coal is removed by loader or excavator and hauled by trucks to a coal processing and stockpiling area. Coal is reclaimed via an underground conveyor and railed to Gladstone.