The technical investigation into the cause of the incident is continuing with the assistance of geotechnical strata experts including former University of New South Wales School of Mines head Bruce Hebblewhite but information is still “scanty”, he said after delivering his address to Coal 2015.

“The circumstances of the incident was [that] there was no gas involved. It was all stress driven,” Regan said.

“Some of our greatest experts are investigating the incident.”

Mining restarted at Austar in December but the continuous miner would not be used in the affected area of the mine. Instead, shotcreting would be done where the outburst occurred, Regan said.

Austar was also working with Polish and German mines where similar incidents have occurred to determine the reasons for the collapse, he said.

The NSW Mine Safety Unit investigation into the wall collapse at Austar is examining its design, the incident site geotechnical conditions, and the suitability of engineering and strata controls.

The Unit is also examining the systems of mining and safe work procedures, and the conditions related to the mining activities.

Investigation Unit acting manager Steve Orr said the mine operator was cooperating with the investigation.

An investigation report will be prepared for the Secretary of NSW Trade and Investment and the NSW Coroner.

“Gas outbursts are characterised by the sudden release of seam gas and coal,” Orr said in the report.

“Investigators have examined the mine’s gas monitoring systems and at the time of writing there was no evidence of elevated gas readings before, during or after the event. Further analysis of data and geological conditions is continuing.”

The incident occurred at 9.05pm on April 15. The incident site was in maingate A9, B heading about 25m inbye from the second cut-through. It was about 10km from the mine entrance and 555m below the surface.

“At this depth, the rib and roof strata of the coal seam is subject to significant stress,” the report said.

“This particular heading is in a geologically disturbed zone near an upthrow fault and shear zone. The workers were mining toward the upthrow fault when the incident happened.

“Steel mesh and bolts supported the roof and ribs at the incident site. A combination of steel mesh and chemically anchored roof bolts and cable bolts supported the roof. A combination of steel mesh, chemically and mechanically anchored bolting systems supported the rib. The extent and nature of the bolting is yet to be confirmed.”

At the time of the incident, seven workers were operating a bolter miner and shuttle car to develop a gate road for a future longwall panel.

Two of the workers were on the left hand side of the bolter miner when a major burst of coal from the rib occurred.

Rib material engulfed both men, who died at the scene.