Workers at BHP Billiton's
About 3,500 unionized workers BHP Billiton-Mitsubishi Alliance (BMA) went on strike at 6 pm local time (0800 GMT) in response to what the union said was management going back on in-principle clauses previously agreed to with its workforce.
The action extends a months-long dispute over working conditions in which workers have staged rolling work stoppages since mid-2011.
The current strike may extend into next week, the union said.
BMA-operated mines have a combined output capacity of more than 58 million metric tons per year of mostly metallurgical coal, representing about a fifth of annual global trade.
BMA has reached an impasse with the unions and believes it's time for employees to again have their say, a BHP spokeswoman Fiona Martin said.
An employee ballot will be organized sometime towards the end of April and that will provide guidance on the way forward.
BHP held an employee ballot late last year as well, but the proposed agreement was voted down.
The union has been pushing for greater job security and more pay for its members as rising commodity prices boost mining sector profits.
($1 = 0.9503 Australian dollars)
(Reporting by Rebekah Kebede; Editing by Himani Sarkar)