The NSW Resources Regulator said the ply fell from between roof bolts and that it was still investigating the incident.
A Centennial Coal spokeswoman confirmed to Australia's Mining Monthly that as a result of the incident underground at Clarence Colliery, two employees were transported to hospital by air ambulance.
"One of those injured is still being cared for in hospital," she said.
"The safety and wellbeing of our people remains our priority as we cooperate with the authorities investigating this incident."
Operations at Clarence have not been suspended except in the area where the incident occurred.