The Berejiklian government is investigating an ambitious plan to convert open-cut coal mine pits into a vast lake system the size of "multiple Sydney Harbours" to drought-proof parts of NSW.

Three pits in the Hunter Valley are ready to be turned into artificial lakes and, as more mining operations end over the next 10 to 20 years, more voids will become available.

The Upper Lusatian Biosphere Reserve in Germany has been created from disused coal mines. Credit:Getty Images

The proposal is modelled on Europe's largest artificial lake system in eastern Germany, where old coal mine pits have been turned into recreational lakes, with cycling paths and watersports.

The Lusatian Lake District, which has become a tourist destination after the mining wasteland was turned into a "water land", also provides drinking water to Berlin.