A Turnbull government plan to dump 2600 Olympic swimming pools of rubble in Alpine lakes risks "catastrophic" damage to Australia's most important river system if it goes awry, experts and environmentalists have warned.

The plan forms part of the "supercharged" Snowy 2.0 proposal announced by Prime Minister Malcolm Turnbull last year. It would expand the Snowy Hydro project to secure electricity supplies to the east coast market, acting as a giant battery and increasing generation capacity by 2000 megawatts.

A plan to dump 2600 Olympic swimming pools of rubble in Alpine lakes risks "catastrophic" damage to rivers, environmentalists and experts say. Credit:Ross Caddaye

The project involves two large underground caverns and a 27-kilometre tunnel linking the Tantangara and Talbingo reservoirs, which would be bigger than the Sydney Harbour crossing and the Melbourne Metro project.

A feasibility study released last month said the project is expected to extract 6.5 million cubic metres of compressed dirt and rubble, known as spoil.