Police say it could be days before the bodies of two men can be recovered after a coal mine collapse in the New South Wales Hunter Valley.

Philip Grant, 35, of Metford and 49-year-old Jamie Mitchell of Aberdare were killed last night while working 500 metres underground at the Yancoal-owned Austar mine at Paxton, south of Cessnock.

They were working with five others about 500 metres below the surface and 10 kilometres from the mine's entry point when a wall collapsed about 9:00pm (AEST).

Mine rescue workers spent most of the day assessing the site of the wall collapse.

The general manager of New South Wales Mines Rescue, Paul Healey, says recovering the bodies will be a delicate process.

"It's very very important that we look after the safety of the people doing the recovery and people assisting in that process," he said.

Sorry, this video has expired Hunter Valley town rocked by mine deaths ( Ben Worsley )

"We've been to the site and we can see what needs to be done but there's still quite a deal of coal to be moved in the immediate vicinity.

"That will be occurring as soon as the risk assessment and safe work method statements have been developed."

Earlier Superintendent Garry O'Dell said the other workers in the area at the time of the collapse were able to escape.

"The other miners attempted to render assistance to those men and unfortunately were unable to do so ... they had to come out of the mine due to their own safety as well," he said.

It is understood they suffered no major injuries.

The cause of the wall collapse is under investigation by the Department of Resources and Energy as well as police.

Mining operations have ceased for the day as management informs workers of the incident.

A crime scene has been established and police will prepare a report for the Coroner.

Mine has a 'chequered past': CFMEU

The Construction, Forestry, Mining and Energy Union's (CFMEU) Northern District president for mining and energy, Peter Jordan, said the incident occurred while a crew were operating a continuous miner.

"There was a massive rib failure where a burst of coal came out from the rib - the wall of the mine - and buried part of the continuous miner and in doing so, buried two of the mine workers," he said.

Mr Jordan says he understands the mine has "been operating in a safe manner" despite having a "chequered past".

"There's nothing to immediately cause a concern to say that the mine was unsafe ... it's a difficult mine, has been for many years," he said.

"Back in and around 2000, under previous ownership, [the mine] caught on fire and it was closed."

He said the mine remained shut for many years before reopening under a new owner.

"There's certainly been some faults here and there but my understanding is that that's been addressed and the mine has continued to work in a reasonably safe manner," Mr Jordan said.

The CFMEU's district secretary, Grahame Kelly, added the mining industry is "highly regulated ... which we think is a good thing".

"There are people within staff whose pure role is to look after the safety of people underground," Mr Kelly said.

But he says for now the union's focus is on the families and colleagues of the two men who have died.

An ambulance sits at the Austar coal mine at Paxton in the New South Wales Hunter Valley. ( ABC TV )

"The matter here, from our perspective, is that a couple of miners have been killed ... our thoughts and prayers are with the families of those two members and also their workmates," he said.

He says more than 1,800 mine workers have been killed in the Hunter Valley coal mines since mining started.

"The good thing is, that in the more recent past, we haven't been adding too many names to the wall," he said.

Yancoal spokesman Reinhold Schmidt says the mine has a strong safety record.

"We do not have any details of what caused the accident. Our focus at this time is on working with emergency services to manage the recovery of the two workers," he said.

"Our thoughts are with the families, colleagues' and friends of the two employees.

"It is really important for us [that] the safety and wellbeing of all our Austar colliery employees are maintained.

"We have been in contact with the families of the workers, offering them support. We've appointed a member of our team to work with these families."