Eight miners were working 500 metres below the surface when the collapse occurred, trapping the two miners.

A 35-year-old man from Metford and a 49-year-old from Cessnock were pronounced dead about 12.15am today.

Six ambulances were on scene and an air ambulance was on standby after the two men were reported missing. Police Rescue, Fire and Rescue NSW, the Vehicle Rescue Association and the Mining Rescue Squad also attended the rescue site.

NSW Police Superintendent Garry O'Dell is quoted by the ABC as saying it will take "some time" before the two men can be released from the mine.

"There is a fair bit of equipment and coal in the area that needs to be removed to be able to get access to them," he reportedly said.

"Ten kilometres along the mine is quite some distance and they need to make sure all is safe along the way, so that's why it is a difficult process both to get to the point, and get the men out."

Austar is a deep underground longwall coal mine located in the Lower Hunter Valley, producing coking coal for export.

Chinese-majority owned Yancoal chose Austar to introduce a new technology called longwall top coal caving (LTCC) technology in September 2006. LTCC technology is ideal in thick coal seams and enables much greater resource recovery in seams such as Austar's Greta Seam.

By using the LTCC mining method, a further 80% of the additional coal in the seam can be accessed and recovered, significantly improving the total recovery of the in-ground coal resource and overall mine economics.

The mine employs more than 409 staff including contractors and was previously known as Southland Coal Mine.

Austar has started production in the new Stage 3 area that it claims will extend the life of the mine significantly.

Yancoal Australia purchased Austar Coal Mine in December 2004, changing its name from Southland to Austar in the process.

The underground coal mine recommenced commercial mining operations in 2006 as part of a $250 million capital investment program that was meant to ensure long-term viability and safety.

The large tenement near the current mine workings and mine infrastructure is relatively unexplored and has the potential to contain significant additional resources.

On its website, Yancoal states that the health and safety of its people are its priority.

"The Health, Safety and Environment Committee assists the Board in overseeing Yancoal Australia's health, safety and environmental responsibilities," it said.

"Our sites use the latest safety management systems, such as the SAFEmap Safety Management System, for which Proactive Hazard identification and extensive hazard and incident investigation are fundamental.

"Our emphasis on personal protection and safety apparel is matched by incident and emergency preparedness."