The Resources Regulator found the winder counterweight was misaligned, causing it to strike a water ring 240m below the surface and rip it from the shaft wall.

 

Chief compliance officer Anthony Keon said the regulator's causal investigation into the incident also identified a number of other factors that led to the workers becoming trapped in the No.3 shaft friction winder on the afternoon of September 5 2018.

 

"The workers became trapped when the lift carrying them up to the surface suddenly broke down about 160m short of the surface following an earlier collision between the winder counterweight and the side wall of the shaft," he said.

 

"The water ring became wrapped around shaft services, winder head ropes and balance ropes, pulling them out of alignment and tripping the winder protection circuit - stranding the workers for more than eight hours."

 

A contributing causal factor was found to be uneven tension on the ropes holding the counterweight, which caused it to rotate and come into contact with the shaft wall while it was moving.

 

"The team also found monitoring of the rope tensions wasn't fully covered by maintenance and inspection, and some of the early indicators of the problem that caused the incident were not communicated to senior management to action," Keon said.

 

"We set up a team including people from Tahmoor Colliery, Shipping, Infrastructure, Mining, Energy and Commodities, the Construction, Forestry, Maritime, Mining and Energy Union and the Resources Regulator.

 

"The Resources Regulator has produced a detailed investigation report and animation of the incident and is calling on mine operators immediately to review their safety management systems in light of the lessons learned."