It was such a good idea, it just could not wait.

It has been revealed Snowy Hydro first put the concept of "Snowy 2.0" to the Prime Minister's Office less than two weeks before the project was announced by Malcolm Turnbull.

The project, with an estimated cost of about $2 billion, is the subject of a feasibility study due later this year.

That figure could become over $3 billion, once the cost of new power lines to transmit the power into New South Wales and Victoria is established and factored in.

During a Senate Estimates hearing, Greens senator Larissa Waters asked Snowy Hydro chief executive officer Paul Broad for a timeline on the announcement, including the first interaction with the Prime Minister's office.

"There were discussions with the Prime Minister's office I think on the 3rd or the 5th of March," he said.

"And the announcement was on the 16th of March."

Mr Turnbull travelled to the Snowy Mountains to make the announcement, saying the Government would bring the "leadership and money" required to make the expansion vision a reality.

Snowy Hydro met with the Australian Renewable Energy Agency (ARENA), which is working with Snowy Hydro on the feasibility study, on February 24.

The NSW Government, which own 58 per cent of Snowy Hydro, was briefed on the plans by Snowy Hydro on the Monday prior to the announcement.

During his opening address to the hearing, Mr Broad said the concept of pumped storage was not new, noting it was the subject of a memo to the federal government in 1966.

But he said the new initiative did not come into frame until quite recently, prompted by significant changes in the electricity market.

"The question of pumped storage really didn't get back on our radar until about 12 months ago," he said.

The decision to take the idea to ARENA was partly prompted by successful funding applications made by other agencies for pumped hydro projects.

Additional billion-dollar cost for new power lines

Snowy Hydro has estimated the total cost of the expansion project at $2 billion, but describes that figure as a "very rough, top-level estimate".

Under questioning from Senator Waters, Snowy Hydro chief operating officer Roger Whitby said the figure did not include costs for transmission lines.

"Other than the direct connections, there is no deeper transmission augmentation that's included in that estimate," he said.

"And for clarity, for this to be feasible more broadly and providing benefits to consumers, it will have to have deeper transmission augmentation, both into NSW and Victoria."

Questioned on the cost of those additional lines, he said it was still being evaluated and was very uncertain.

"That is still the subject of ongoing work, we're working closely with Transgrid on the initial part of that at present," he said.

When pressed, he said the cost was potentially over $1 billion.

The two executives later clarified that the cost of transmission would not form part of the feasibility study, and responsibility for transmission will fall to Transgrid.

Mr Whitby said the figure of over more than $1 billion came from early works done with Transgrid on the project.