Malcolm Turnbull's much-vaunted Snowy Hydro 2.0 project will take at least six years to build, depending on a feasibility study and what geological problems are identified.
The prime minister flew to Cooma in a helicopter on Monday to announce a broader feasibility study than initially planned.
In its first stage of construction, the project will see a 2000MW of underground generation and 29km of tunnels between existing reservoirs in the Snowy Mountains region.
"This was a nation-building project and what we are doing now is doubling it," Mr Turnbull said.
At its peak the project will create 5000 jobs, but it is already employing about 350.
Snowy Hydro chief executive Paul Broad said the feasibility study, being done in conjunction with the Australian Renewable Energy Agency, would reveal the timeframe but he expected the project would take six years.
"It depends very much on the geology and we will firm it up in the next month or two," he said.
One of the unknown factors is whether the tunnels will need to be lined with concrete or if they can be stabilised in other ways.
The total spend on the planning phase will be $29 million and be completed by the end of the year.
