More than 400 jobs will be lost with the closure of two power stations and a coal mine in South Australia.
Alinta Energy says it will shut its Port Augusta power stations and the Leigh Creek mine by March 2018 but is also considering pulling the plug as early as March next year.
The company's decision prompted a $1 million commitment from the state government to help local communities amid fears other regional jobs could go.
It also came as unemployment in SA surged to a 14-year high in May of 7.6 per cent.
Workers were told of Alinta's plans on Thursday and have been promised their full entitlements, worth about $75 million.
Premier Jay Weatherill said government ministers would visit the region in coming days and the federal government would also be asked for assistance.
"Alinta is a major employer so this decision will clearly have a profound impact," he said.
Alinta chief executive Jeff Dimery said the decision to close the SA operations came after a detailed review found there was no chance of a return to profitability.
The company had already lost about $100 million despite investing $200 million to extend the life of the power stations and the mine, he said.
"Throughout the four and half years we have been running the business we have investigated all possibilities to find a viable economic solution for its continued operation."
Supporters of alternative energy urged the state government to provide funding for a solar-thermal power plant in Port Augusta.
"Fossil fuels are falling over in South Australia," Conservation Council SA chief executive Craig Wilkins said.
"But the good news is that our renewable energy industry is ready and waiting to step up, replacing our old electricity supply and the jobs that went with it."
The state opposition said the government must use next week's state budget to offer tax relief to businesses to encourage job creation.
"If changes are not made, Alinta Energy will not be the last major business to shut its doors and add to South Australia's ever-increasing unemployment line," Liberal leader Steven Marshall said.
The Australian Services Union said the closures would devastate Port Augusta and urged the state government to develop a jobs plan for the region.
"The Port Augusta community cannot afford a blow of 400 jobs to their community and the South Australian economy cannot afford further job losses," union secretary Joseph Scales said.
"I know of whole families who are employed at the power station - wife, husband, father-in-law. What do these families do now?"
