Bill English told Newstalk ZB this morning that Solid Energy and the government had done everything it could to reduce cost at its Stockton mine and be more productive.
"They've been through a series of changes at Stockton and the coal price just keeps falling away," he said.
"We've been trying to do everything we can to secure the viability of the company and I'll have to say as time has gone on that's become more and more challenging because the coal price has kept dropping.
"The coal market has been struggling globally and Solid Energy was undergoing negotiations to try to assure the company's future, he said.
"In the long-run the price of coal could come back up, Mr English said, but "the swings are pretty big".