Australian coal mines owned by BHP Billiton and Mitsubishi Corp will be hit by a second seven-day strike after a third of the mines' workforce rejected a BHP labour deal, union officials said on Wednesday.

The mines, operated under the BHP Billiton-Mitsubishi Alliance joint venture, have a combined output capacity of more than 58 million tonnes a year, representing about a fifth of annual global trade.

About 3,500 employees led by the Construction, Forestry, Mining and Energy Union (CFMEU) completed a seven-day strike on Tuesday. The union did not give a date for the second strike and union officials were not immediately available for comment.

The strike will not close mines, but again reduce operations. The mines employ around 10,000 people.

On Monday, BHP declared force majeure on deliveries from the predominately metallurgical coal mines in the Bowen Basin, citing long-running labour stoppages compounded by heavy rains that have impeded production runs.

Force majeure is a legal clause relieving companies of immediate supply obligations due to circumstances beyond their control.

Mine workers rejected the latest offer for an enterprise bargaining agreement and overwhelmingly voted to strike again, said CFMEU District President Stephen Smyth.

He said workers rejected the offer because BHP had changed its position on roster issues.

"The workers are ropable (angry) that these negotiations have gone on for so long and just when they thought a resolution was getting nearer, the company comes back to the table having altered in-principle agreement clauses," he said in a statement.

In an email to Reuters, BHP said it was "extremely disappointed" by the union action.

"The company has consistently demonstrated its commitment to negotiating constructively with the unions," BHP said, adding it has already made an offer of a 5 percent a year wage hike for the next three years, plus a guaranteed A$15,000 per year bonus.

The CFMEU said the dispute was not about money, but ensuring better conditions and facilities for miners and their families.

BHP will release its March-quarter production data on April 18. BHP warned in January that labour unrest along with heavy rains in the region were affecting production volumes.

Prices for the metallurgical coal mined by BHP and Mitsubishi have weakened in recent months but are still more than twice as high as estimated production costs at the mines of around $80 per tonne, according to analysts.