440 jobs will be lost after Alinta Energy announced it will close two coal-fired power stations in Port Augusta, along with the Leigh Creek coal mine
Alinta said its Northern and Playford B power stations and Leigh Creek coal mine would not operate beyond March 2018, but could close even earlier.
The company said the operations had become "increasingly uneconomic."
Alinta said the decline in demand for energy, as households have become more efficient and the number of industrial customers has declined, combined with policy settings designed to support significant growth in renewable energy generation, have together had the effect of causing a significant oversupply of power available to South Australia.
Alinta chief executive Jeff Dimery said the company had tried to keep the operations open.
"Throughout the 4½ years we have been running the business we have investigated all possibilities to find a viable economic solution for its continued operation," Dimery said.
"During this period the company has incurred operating losses in the vicinity of $100 million whilst at the same time investing an additional $200 million to extend the operating life of the Flinders business.
"We now believe there can be no expectation that the Flinders business can return to profitability."
Employees have been advised about the decision to close operations, and Alinta said it is working with the SA government, unions, and local communities to prepare for the shutdown.