Origin Energy is no longer reliant on coal from Centennial's Myuna and Mandalong mines to keep Eraring power station operating.
The company confirmed this week its reliance on locally sourced coal had steadily declined since the installation of a rail unloader loop in 2013.
Eighty per cent of the plant's stockpile now comes from Hunter Valley mines. The use of Centennial coal could be reduced to zero without affecting plant operations.
Origin has blamed a combination of cost, quality and reliability issues for reducing its dependence on Centennial's coal.
The company finalised a deal with the NSW government in May to keep the coal-fired generator open for an extra two years, by which time it is hoped enough clean energy infrastructure will be in place to support the grid.
Soon afterwards it struck a deal with Centennial to supply coal to Eraring for a further six months beyond the end of its existing contract.
But it wasn't enough to save miners' jobs at the Mandalong mine.
Centennial said on Wednesday that it was cutting 200 jobs, citing its inability to win a new supply deal with Origin, the state's coal caps and high costs for the decision.
"Workers thought they had a lifeline through the six-month extension which would allow more time to plan and see where the jobs of the future could be," Mining and Energy Union Northern Mining and NSW Energy District president Robin Williams said.
An Origin Energy spokesman said the company remained in negotiations with Centennial regarding the Myuna colliery.
"We're hopeful of reaching an agreement on future coal supply from Myuna at a fair and competitive price that can flow through to consumer power bills," he said.
Australian Manufacturing Workers' Union NSW organiser Tim Jackson said the Mandalong job cuts had left the workforce "gutted".
"I think to find out after the last six to 18 months of concern and asking these questions on a regular basis, to finally find out hits home pretty hard, not only for themselves but those with families, kids and a community in the workforce," he said.
Mr Jackson said the redundancy consultation process with Centennial Coal was expected to start early next week and the unions' intention would be to save as many jobs as they could.
He said there had been discussions about whether the company would be open to offering voluntary redundancies, but there was no guarantee.
Mr Jackson said he understood the goal was to finalise the redundancies by mid-October.
"Any sort of consultation process must be done in the right way and the right manner," he said.
"The view from AMWU is if it takes longer than that proposed period it will, we have to get this right to make sure people have time to digest it, assess where their lives are at to ensure we don't have major hiccups or hurdles as we go through."
Mr Jackson said the last-minute deal struck between Centennial Coal and Origin Energy to keep supplying coal to Eraring power station had given workers some "short-term sureties".
"Where this falls flat is that I think we have waited too long, when I say too long the likes of the Hunter Jobs Alliance and their respective affiliates have been pushing for a local transition authority for a good 12 to 18 months now, maybe even longer," he said.
"I know that has support from the state and federal governments to an extent, but the problem is the time it has taken.
"If it was in place sooner, knowing these discussions about transition have been going on for years, if something was implemented sooner we may have been able to have a different outcome than yesterday."
