The samples have been taken at various depths, are catalogued, and are the result of decades of work.

Wallarah 2 project manager Kenny Barry said the samples showed why coal mining under the valleys of Dooralong, Jilliby and Yarramalong – a key water catchment area for the Central Coast – posed no threat to the water supply.

The key, he said, was the presence of aquatards in the geology. Multiple layers of them. And the depths at which the mining would occur.

“We’re starting at almost 400 metres and progressing out to the south-west to 600 metres,” he said.

“An aquatard is essentially an impermeable barrier between the surface and the coal seam,” Mr Barry said.

These impermeable layers were significant because after the mining process, water could not flow from the surface down into the mining void, he said.

Mr Barry said fracturing of the roof of a mine void was common, and well understood.

“But the protections [at Wallarah 2] are depth. It cannot fracture through to the surface. The depth is too great,” he said.

Add to that the presence of impermeable layers of rock above the deep mining area, and the mine provided multiple layers of protection, he said.

“These will absolutely stop that fracturing. The water can’t get down there. That’s a fact. That’s coming out of the modelling.”

Mr Barry said the area targeted by Wallarah 2 had a better geology and was far deeper than the coal mined at Mandalong, yet Mandalong “had no issues”.

He said Chain Valley and Myuna collieries were mining under Lake Macquarie.

“But the water doesn’t pour out of the lake into the mine. That’s the reality.”