Board member Joseph Leonard has been named to succeed Calder as interim CEO.

Calder, who will also step down as a director for the producer, had been interim CEO from March 2010 to March 2011.

Walter officials have retained a third party to search both internally and externally for a permanent CEO.

"I do believe Walter Energy has immense potential and I am sure it will be realized. That said, it is now time for me to move on and pursue other interests," Calder said in his resignation statement.

Non-executive chairman Michael Tokarz said Walter had accepted Calder’s resignation.

“We thank him for helping us through the transition and integration of Walter Energy and Western Coal into one company – the new Walter Energy,” he said.

“We are confident that the company, under the direction of Joe Leonard, will continue its current momentum.”

Walter’s $US3.2 billion acquisition of Canadian producer Western Coal, announced last November and completed earlier this year, created a North American and global steelmaking coal giant – the top publicly traded pure play metallurgical coal producer in the world with access to the Pacific and Atlantic seaboard markets.

In total the producer now owns 385 million tons of reserves, of which 20Mt or more is expected to be mined annually by next year.

Its production profile is balanced between Walter's high productivity assets and Western Coal's high growth assets in Canada, the US and the UK, and it will be the only producer with cost-advantaged transportation access to the Asian and South American seaboard metallurgical coal markets.