An extensive study of the impact on groundwater from Hume Coal's proposed coalmine at Sutton Forest has warned of a drop in the water table of at least 120 metres near the mine, according to the Southern Highlands Coal Action Group.
The study predicted water bores in the 89 square kilometre mine licence area would be drained and the impact would extend over an area of 150 to 200 square kilometres.
Spokesman for SHCAG Peter Martin said the study, which had been peer reviewed, demonstrated the mine proposal was unsustainable and would have a massive impact on the viability of irrigated farming in the region.
"The groundwater drawdown that this mine will take is extreme. It is right out of the box and clearly unacceptable," Mr Martin said. "The impacts on farm bores could be felt within two or three months of the mine starting up."
Hume Coal is expected to release its mine plan in the latter half of 2014.
On April 9, the company cast doubt on SHCAG's report, accusing the anti-mining group of fudging its figures.
At the time, Hume Coal project director Greig Duncan said SHCAG had incorrectly based their water study on a large-scale, wide-panel longwall operation.
"Our experts have advised that the predicted impacts on the region's groundwater would only be plausible assuming a large-scale longwall mining operation," Mr Duncan said.
"This assumption is . . . not something being contemplated in our mine planning."