The Western Australian government says it is disappointed that one of the state's last remaining coalmines will shift to day operations only.
The ABC has received confirmation from the government about Premier Coal's decision to reduce operating hours.
The government is seeking more information on the matter.
Transitioning to day shift operations would result in job losses at the mine in Collie, 200 kilometres south of Perth.
The ABC understands Premier Coal has told workers night shifts will end in the coming months and there will be a three-week planned shutdown over summer.
WA Premier Roger Cook speaks on ABC radio on Thursday. (ABC News: Andrew O'Connor)
"Premier Coal's decision to move its operational workforce to day shift is very disappointing," a state government spokesperson said.
"The government has sought urgent clarity from Premier Coal on the impact these changes will have on its workforce."
Premier Coal did not respond to questions from the ABC.
Unions said the move would mean the axing of about 40 per cent of the workforce, or about 150 jobs.
Management today handed out forms for workers to nominate for voluntary redundancies.
A form asking for voluntary redundancies was handed out at Premier Coal in Collie. (Supplied: AMWU)
The decision comes as owner Yancoal has been embroiled in a tumultuous period of enterprise bargaining with workers as they try to negotiate their final agreement before the government's planned shutdown of coal-fired power generation by 2030.
The workforce, which is more than 90 per cent unionised, has taken several hours of strike action since July, including refusing to load coal onto trains.
Unions say the proposed offer from Premier Coal could leave workers tens of thousands of dollars worse off each year. (ABC South West WA: Neve Brissenden)
Australian Manufacturing Workers' Union Secretary Steve McCartney said the impacts of the decision were not just about staffing.
"It looks like about a 40 per cent reduction in workers," he said.
"That's a lot of people that would be looking for a job where there isn't one at the moment.
"But more importantly, the redundancies will all be paid out on day shift rates and the night shift component will be taken out … that's 45 per cent of their money going missing when they're leaving."
In April, Premier Coal announced the loss of between 70 and 100 jobs at the Collie mine site.
No power blackouts expected
Energy analyst Tim Buckley, from Climate Energy Finance, said the night operations shutdown would not cause rolling blackouts.
Premier Coal's open pit is near Collie in WA's South West. (ABC News: Anthony Pancia)
"Customers of WA can be confident there's not going to be significant impacts to the energy grid from this," he said.
Coal generated 24 per cent of all electricity in WA in the past 12 months, while renewables contributed about 40 per cent.
Mr Buckley said the decision by Premier Coal came sooner than the government was prepared for, but renewable projects across the state were steaming ahead.
In a statement, the state government said it was confident Premier Coal's decision would not affect the state's power supply.
Workforce in transition
Taxpayer money has been pouring into Collie since the government made the announcement about shutting coal production in 2022.
The WA government's $230 million Collie Industrial Transition Fund has laid the groundwork for several new industries, but few have reached a final investment decision.
In May, the WA government also flagged Collie as a potential home for a new manufacturing defence hub.