Bob Young, one of the water experts at Pool N during the February 13, 2014 visit. Credit:Stuart Khan

"To see all of that water was gone … was a big surprise," said Peter Turner, the National Parks Association-nominated member on the Metropolitan Colliery's community consultative committee. He blamed longwall mining in the area for the subsidence and fractures of the pool's floor. Longwall mining involves giant machines that shear coal from the seam which can then cause the rock above to collapse behind the machine as it moves forward.

A spokeswoman for the Sydney Catchment Authority said it was aware of the impact to Pool N but that "there is no evidence to date to suggest that mining activity has affected storage levels in Woronora Reservoir in a substantial way".

A spokeswoman for Planning and Infrastructure said "strict conditions around water management were part of the consent conditions for this project", and Pool N was "upstream of where the specific environmental protections detailed in the consent begin".

Dr Turner said the location of the pool was overridden by the requirements that the mine's longwall expansion had only a "negligible reduction to the quality or quantity" of the water reaching Woronora. There was concern that authorities had not been able to identify where the water was going, nor if the impact was negligible, he said.