Four men have been buried under rubble hundreds of metres underground, at a mine near Picton on the outskirts of Sydney’s south-west.

Investigations are under way into what caused part of the roof to collapse.

The group became trapped about 3am deep below the surface of GM3’s Appin mine at Douglas Park.

NSW Resources regulator Anthony Margetts said they were 400 to 500 metres underground.  

“This incident involves the sudden collapse of a roof directly above where these workers were working,” Margetts said.

It’s believed one man managed to dig himself free, and the others were pulled out after spending an hour partially buried.

Two of them were taken to Wollongong Hospital, the others to Liverpool.

All escaped with non-life threatening injuries.

NSW Ambulance Inspector Dylan Snape said one of the men, aged in his 50s, had head and pelvic injuries.

Multiple agencies are investigating what went wrong.

All operations at the mine have been paused until further notice.

In a statement, the Mining and Energy Union said: “No stone will be left unturned in determining the source of the incident.

“All workers have the right to safely go home at the end of their shift.”

Employment Minister Murray Watt said: “All Australians are thinking of these workers, their families, and their colleagues.

“It’s another reminder that all Australians deserve a safe workplace.”

The Appin area is no stranger to the risks of a mine site.

In 1979, 14 men were killed and dozens were severely burned after a methane gas explosion.

Margetts said it was not clear what caused today’s incident.

“It’s challenging to determine whether this incident is comparable to others, or if it is an unusual occurrence, until we fully understand the specific causes behind this event,” he stated.