Expansion of South32's Dendrobium coal mine could cause cracking in the walls of the Cordeaux and Avon Dams, WaterNSW has said.
In a remarkable submission to the major projects planning process, WaterNSW - the agency tasked with protecting the sate's water resources - has voiced strong opposition to the expansion, which would mine under the drinking water catchment Special Area.
"There is the potential for differential far field horizontal movements on the existing Avon and Cordeaux Dam walls, which could cause cracking in the walls," WaterNSW's submission states.
"This has not been adequately addressed in the EIS.
"While the proposed longwalls are setback at least 1km from both Avon and Cordeaux Dam walls, the mining company has not adequately considered the potential for differential far field horizontal movements.
"The structural integrity of the dam walls is critical to the ongoing reliability of water supply to the people of Sydney and the Illawarra and the safety of people living downstream of the dams."
There is the potential for differential far field horizontal movements on the Avon and Cordeaux Dam walls, which could cause cracking in the wallsWaterNSW
WaterNSW said the predicted impacts from mine were "unacceptable" for the drinking water catchment.
The mine would cause subsidence higher than any recorded figures of any other mine in the Southern Coalfields region, with up to 3.3 gigalitres of drinking catchment water per year lost into mine workings through cracks, it said.
The Mercury sought comment from South32, which declined a detailed response while it prepares its formal response to submissions.
"The EIS reflects that we will not mine under water supply reservoirs, named watercourses and key stream features," a South32 spokeswoman said.
The Dams Safety Committee had recommended any longwall mining not come within 1500m of undermining dam walls. South32 proposes a 1000m setback.
WaterNSW said this was the first mine considered under the "neutral or beneficial" (NorBE) test as required by 2011 law for drinking catchment mines. It said the NorBE assessment in the EIS was inqadequate.