The mine has been out of action since March 18 after the NSW Resources Regulator inspected and found safety issues were too serious for underground work to continue.

There had been a series of roof collapses leading up to this decision.

But another roof fall was discovered on the main belt road when mine staff went underground on April 1.

This has forced the mine's hand, and a reduced workforce will now focus on recovering equipment, before placing the operation into "care and maintenance" mode.

"The mining operations at the Wongawilli colliery will be suspended and placed into 'care and maintenance', which will result in the reduction of the workforce by approximately 45 employees," Wollongong Coal told the ASX on Wednesday.

"The operational changes will affect operators, electricians, fitters, deputies and other staff.

"A small team will be retained to undertake care and maintenance activities."

The company blamed "aged infrastructure and equipment" for the difficulties at Wongawilli, which had caused Wollongong Coal to sustain "significant losses".

Bob Timbs from the Construction, Forestry, Mining and Energy Union said the workforce had been informed on Wednesday of the plans for Wongawilli.

"They're going to recover whatever equipment they can," he said.

"Some CAS Mining employees will remain at the site. They're looking at redeployment options to other mines and/or Russell Vale."

He said dozens of workers had been redeployed already to the Russell Vale mine, where Wollongong Coal was confident it would be able to resume mining once planning approvals are given. 

This had not been an easy task, with significant inadequacies identified in the assessment of the mine's environmental impact on the water catchment.

Russell Vale has been under care and maintenance mode since 2015.

Wollongong Coal said it expects its underground expansion plans for Russell Vale to be referred back to the Planning Assessment Commission for determination "later this year".