The National Parks Association says developers of power lines connecting the Snowy 2.0 pumped hydro project to the grid ignored alternatives despite the pylons cutting through a national park.

In an open letter to the Berejiklian government, the association says TransGrid should have detailed other options in its environmental impact statement to the building of two giant 330kV overhead transmission lines. The EIS is near completion.

The letter, signed by present and former engineers and also two dozen environmental groups, said the lines would traverse nine kilometres of the Kosciuszko National Park and adjacent Bago State Forest and require clearing an easement of as much as 200 metres in width in the pristine region.

"Overhead transmission lines are incompatible with our obligations for the protection of Kosciuszko National Park, the Australian Alps National Heritage Place, the UNESCO Kosciuszko International Biosphere Reserve, Aboriginal cultural heritage, head-water catchments for south-east Australia’s major rivers, unrivalled natural landscapes, and unique biodiversity," the letter said.