Coal mining in the Hunter Valley has affected groundwater in about a quarter of the region, and the 22 planned new coal mines or expansions of existing ones will expand the impact on water resources further, a federal government study has concluded.

The Hunter bioregional assessment, published online this week, examined the expected impacts of the extra mining in an area "of great ecological significance".

Hunter Valley coal mines are affecting groundwater in the region – and the impact will grow as mines expand. Credit:Michele Mossop

The report found the existing 40-plus open-cut and underground coal mines had affected an area of 4307 square kilometres, or about a quarter of the overall Hunter region. The 22 new projects would expand that impact – assessed as at least of a 5 per cent chance of groundwater levels dropping 0.2 metres – out to about 30 per cent.

The effects on groundwater diminish with distance from mines. Still, the report found it was "very likely" that at least 35 square kilometres would experience a drawdown of at least five metres from the new mines, while an area of at least 121 square kilometres would endure at least a two-metre drop.