Respected coalminers and fathers Jamie Mitchell, 49, and Phillip Grant, 35, are believed to have been killed instantly when a 15-metre wall of coal erupted and fell onto them as they worked on preparing a new longwall at the Yancoal-owned Austar mine on Tuesday night.

The mine remained closed on Wednesday night as rescue teams worked to determine the safest way to the site of the accident, which is about 500 metres below the surface and 10 kilometres from the mine opening.

The recovery operation could take several days and the mine could be shut for weeks.

Mr Mitchell, an Aberdare-bred man who was the son and father of miners, and Mr Grant, of Metford, were part of a crew of seven using a continuous miner to gorge out new areas of the site ready for longwall mining when tragedy struck. Two of their workmates watched helplessly as the massive wall of coal rained down on them.

The five uninjured miners called for help before frantically trying to make the area as safe as they could for any rescue operation.

University of Newcastle's Professor Garry Willgoose said the work being carried out by Mr Mitchell and Mr Grant was the most risky at the mine.

He said this was partly because a section of up to 20 metres at the front of the new tunnel, above and directly behind where they were drilling, could not be immediately secured by rock bolts.

''The development stage, where they were are effectively drilling tunnels for the construction of a longwall, is the most dangerous. They don't know the exact details of all the geology as they are drilling.''

Professor Willgoose said pressure bumps, or rock bursts, were not ''uncommon'' in the ''types of geology'' the miners were working in, but they were difficult to predict. ''I am told by people who are familiar with that mine that this has happened before,'' he said.

Yancoal Australia chief executive Reinhold Schmidt said the company would work closely with regulators and emergency services.

''Our thoughts are with the families, colleagues and friends of the two employees as well as the wider community in the Hunter Valley at this very difficult time,'' Mr Schmidt said.

''We are unable to confirm the cause of the incident and it would be inappropriate to speculate.''

Construction, Forestry, Mining and Energy Union northern district president Peter Jordan said the survivors had met union officials on Wednesday as they dealt with the loss of two workmates.

''It is having a major effect on the mine workers, on the mining community and a wider effect on the community as a whole,'' Mr Jordan said.

He said the union was not aware of any safety concerns at the mine. ''There has been nothing that has been raised our concern in the immediate past,'' he said.

Minister for Resources and Energy Anthony Roberts extended his condolences to the families, colleagues and Hunter community rocked by the deaths.

Mr Roberts said the incident could lead to widespread reform in the state's mining industry.

Mr Grant was an avid swimmer who enjoyed watching his son Shaun swim laps at The Forum at the University of Newcastle. He had been a long-time member of The Forum's adult swim squad.

Squad coach Michale Pfeiffer said Mr Grant was ''an extremely dedicated father'' who had taken delight in Shaun's interest in swimming and BMX pursuits.

''He … did everything for Shaun,'' Mr Pfeiffer said.