Mine owner Peabody Energy had halted coal extraction operations at the long-running mine early in January, saying demand and prices for coal were weakened under the pandemic.

The approximately 250 workers were then dealt another blow in February, with the US-owned miner deciding to extend the closure for another two months.

A spokeswoman for Peabody told the Mercury workers would resume duties as per their shift rotations, starting from night shaft from 6pm tomorrow.

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"We again thank our loyal employees and their families for their patience, understanding and ongoing commitment to the mine," she said.

"The safety of our workforce is always Peabody's first priority and Metropolitan will continue to operate under the highest of health and safety standards, systems and procedures."

Now negotiations for a new Enterprise Agreement will continue and Bob Timbs, district vice-president for the Construction, Forestry, Maritime, Mining and Energy Union, said two versions for an agreement had already been put to the workforce, and been voted down unanimously both times.

He called for Peabody to return to negotiations in good faith.

Similar shutdown periods at Peabody's Wombo mine in the Hunter Valley were swiftly followed by massive redundancies but Peabody has insisted throughout that this was not the plan at Helensburgh.

"The guys are looking forward to returning to work - they've been through some hard times for the past four or five months," Mr Timbs said.

"They'll be happy about getting back to work."

The mine has been in "care and maintenance" mode which means a small workforce remained on site to keep the underground operations in working order.