The Green Party is worried a partial sale of Solid Energy, could lead to a rapid expansion of environmentally harmful mining practices.
The State owned coal company, along with Meridian, Mighty River Power, Genesis and Air New Zealand will be partially sold if National is returned to Government.
Solid Energy is welcoming the prospect and says it will help provide the large amounts of capital it needs to build a plant in Southland to turn lignite coal into diesel.
But Green candidate Eugenie Sage says this will have a disastrous impact on the environment and massively increase the country's greenhouse gas emissions.
Ms Sage says Solid Energy should instead focus on environmentally-friendly activities such as the production of wood pellets.