The bill for the Snowy Hydro expansion could be twice the initial estimate, while the project's delivery time frame has been increased by two years.

Executives from Snowy Hydro Limited have estimated that an essential upgrade of power transmission lines from the mountains into Sydney and Melbourne will cost up to $2 billion, effectively doubling the cost of the total project.

When Prime Minister Malcolm Turnbull announced the "supercharged" pumped-hydro scheme in March, he estimated it would take four years to dig 27 kilometres of tunnels and sink a turbine a kilometre below ground to make the project ready to plug in to the National Energy Market.