However, the operator said it could not confirm the exact recommencement date in its communication to employees required under the Worker Adjustment and Retraining Notification Act of 1988, or WARN Act.

The notification is required because the Act considers a closure of more than six months an "employment loss" and that employees must be given notice.

The mine was evacuated and idled in July after a series of roof falls in mined-out sections that damaged portions of its ventilation system. January would mark six months since its closure.

"We are uncertain when we will be authorised to re-enter the mine and resume work," officials said in a statement.

Added company external affairs senior vice-president Thomas Hoffman: "There was no catalyst for this other than the recognition that we are now four months into the idling and the plans for re-entry have not yet been finalised. The update on Buchanan that we provided on October 25, 2007 remains our current view of the situation at the mine."

The company said in its third-quarter report last month that the idling of the mine has cost it $US84 million since July in atmospheric management and monitoring expenses as well as lost income from sales after stockpiles at the operation emptied.