The New South Wales Resources Regulator said in a causal investigation of the May 3 incident that the catastrophic failure of the underground bulkhead seal caused a rapid loss of drilling mud from a blind bore drilling shaft.

"The mine had foreseen the risk of inrush and to eliminate or minimise the risk had introduced an inrush control zone underground while work was being undertaken to sink the shaft," it said.

"The inrush control zone prevented workers from being exposed to the fatal hazard that ultimately occurred."

Within minutes of the drilling mud escaping from the shaft, the alluvial materials close to the surface began to collapse into the shaft void, due to the rapid loss of confinement pressure.

A sinkhole rapidly developed and the drill rig personnel moved to a place of safety, before both the drilling rig and constructed drilling pad partially descended into the sinkhole.

Subsequently, an underground inspection was undertaken by mining supervisors, who confirmed there was clear evidence the bulkhead had catastrophically failed, and that no further source of ground instability was identified.

In its report, the regulator found the risk assessments conducted for the project had not considered the impact of the rapid loss of confinement pressure created by the rapid loss of drilling mud on the stability of the upper section of the shaft being drilled.