Before work on the Snowy 2.0 energy project has even begun, scientists are voicing concerns about proposed exploration works — including "contentious" plans to dump spoil in a reservoir.

Details are contained in an environmental impact statement (EIS) currently on public exhibition and being assessed by the NSW State Government.

In it, Snowy Hydro Limited outlines its intention to blast and drill a 3.1 kilometre tunnel to test rock conditions in preparation for designing a subterranean power station.

Exploratory works will include an underground tunnel. ( Supplied: Snowy Hydro )

The station is a key feature of the Snowy 2.0 pumped hydro-electric project and would sit 1 kilometre underground between two linked dams — Tantangara and Talbingo.

Scientists like Jamie Pittock, an associate professor from the Fenner School of Environment and Society at the Australian National University, are worried about the tunnel project.

"This EIS for the exploratory work is going to test one of the more contentious elements, which is the deposition of construction spoil in the lakes," Dr Pittock said.

"It's good that they are testing that to see what the impact is at a small scale before going ahead at a larger scale," he said.

Proposed site 'already extensively disturbed'

To build the tunnel, 750,000 cubic metres of dirt and rock would have to be extracted.

Snowy Hydro then plans to sink geochemically "non-reactive" spoil into Talbingo Reservoir — starting with about 50,000 cubic metres as a trial.

Scientists fear Redfin fish could destroy native fish. ( Supplied: Fishonfly )

Dr Pittock also questioned what plans were in place to stop the "invasive" Redfin fish in the lower river being pumped up into Tantangra Reservoir "because we don't want that nasty predatory fish chomping all our native fish and wildlife up on the main range".

Dr Pittock and other academics are calling on Snowy Hydro and the Federal Government to look at rehabilitating the remaining damaged areas from the construction of the original Snowy Hydro scheme and to ensure environmental flows are better released down the Snowy River.

"The areas where the Snowy 2.0 development is proposed are already extensively disturbed, for example, where this access tunnel is proposed is at the site of a former copper mine," he said.

"So the main concern is really about how the developers offset the damage they're causing to a national park by investing in betterment in other places in the park."

In a statement, Snowy Hydro CEO Paul Broad said every effort was being made to avoid and minimise environmental impacts, but those that could not be avoided would be fully offset.

Mr Broad said Snowy Hydro was working closely with CSIRO, Royal HaskoningDHV and Cardno on the excavation of the rock and with Charles Sturt University and NSW DPI Fisheries to determine the likelihood of fish — including Redfin — surviving the journey through Snowy 2.0 and preventing widespread transfer.

Snowy 2.0 was initially estimated to cost $2 billion. However, Mr Broad has previously said the price tag could rise to between $3.8 and $4.5 billion.

The company will start tunnelling at the end of this year if approval is granted.