Pressurised gas has been ruled out as the cause of a rockfall that killed two miners at the Austar underground coal mine last month.
By examining the gas monitoring system, investigators at the Yancoal-owned mine have found that there was no evidence of higher gas readings before or after the accident.
This means that the coal burst could not have been caused by gas pressure as was surmised by some following the accident.
The workers involved were operating a bolter miner and shuttle car, with bolting rigs on the miner.
Grant and Mitchell were on the left hand side of the bolter miner when a burst from the coal rib caused a section of the rib, which was supported with various lengths of bolts and mesh lost confinement and moved into the roadway where the material engulfed the two men.
Five other workers involved with work at the time were unharmed, and attempted to rescue the men, however the area was deemed unstable.
The Mine Safety Investigation unit report said that the incident occurred 10 kilometres from the mine entrance and 555 metres underground, where rib and roof strata of the coal seam is subject to significant stress.
The investigation found that steel mesh held with chemical bolts and cable bolts supported the roof and ribs of coal at the accident site, however the extent and nature of the bolting is yet to be confirmed.
The investigation is presently examining the design of the mine, geotechnical conditions and the suitability of engineering and strata controls, as well as the systems of mining and safe work procedures in place at the time.
Pictured is the site of the accident after the two bodies were recovered.