Prime Minister Malcolm Turnbull’s “supercharged” Snowy Hydro plans will cost close to double the initial estimates.

Executives from Snowy Hydro Limited have to senate estimates that the upgrade of power transmission lines from the mountains into Sydney and Melbourne - an essential part of the ‘Snowy 2.0’ scheme - will cost up to $2 billion.

When the improved pumped-hydro scheme was announced in March, the Federal Government estimated it would take four years to dig 27 kilometres of tunnels and sink an underground turbine to connect the project to the National Energy Market.

Snowy Hydro says it will actually take closer to six years due to the “challenging” geology involved.

But the “deep transmission” upgrade is needed because it is such a long way between the chosen dams and the large population centres they will power.

The hearing earlier heard that Mr Turnbull's announcement came just a fortnight after discussions first started between his office and Snowy Hydro.

The Australian Renewable Energy Agency, which is involved in distributing funding for the new scheme, did not have its first “substantive discussions” with Snowy Hydro until February 24.

Snowy Hydro has started its $29 million feasibility study, which is expected to produce a better picture of the challenges and costs.