Engineers, economists, energy specialists and environmentalists are calling for a final decision on the Snowy Hydro 2.0 project to be delayed to allow an independent review, claiming it will cost far more and deliver far less than has been promised.

The group of 30 said the 2,000-megawatt pumped hydro storage project in the Snowy Mountains would permanently damage the Kosciuszko national park.

“Snowy 2.0 is not as it has been publicly portrayed,” they said in a joint statement. “There are many alternatives that are more efficient, cheaper, quicker to construct and incur less [greenhouse gas] emissions and environmental impacts.”

The group was assembled by the National Parks Association of New South Wales and Ted Woodley, a former managing director of Energy Australia. In a letter to the prime minister, Scott Morrison, and NSW premier, Gladys Berejiklian, the members said they hesitated to raise the issue during the unparalleled challenge of the coronavirus pandemic, but they were concerned about the project going ahead without “independently validated justification”.

They said they were sceptical about the merits of Snowy 2.0 when it was announced by former prime minister Malcolm Turnbull in March 2017 and that scepticism had consolidated as more information emerged.

Their listed concerns included that they believed:

About 40% of the energy generated is lost before it reaches consumers, more than other pumped storage schemes due to the distance between reservoirs being far longer, and more than other storage options.

It was likely to cost at least $10bn, compared with Turnbull and Snowy Hydro’s initial estimate of $2bn. A $5.1bn contract has been signed as part of the project, with further costs to be added. They say this will be more than the Snowy Hydro’s estimate of the market benefit of between $4.4bn and $6.8bn.

It would require substantial transmission works to connect to the grid, costing billions more.

It will lead to more than 50 million tonnes of carbon dioxide emissions during construction and the first 10 years of operation.

It will convert extensive areas of national park into a construction site, with permanent damage over thousands of hectares and the destruction of habitat used by 14 threatened species.

“Snowy 2.0 should not proceed on the basis of overstated claims that have never been tested,” the letter concluded. “At stake are billions of dollars of Australian taxpayers’ money, tens of millions of tonnes of greenhouse gas emissions and thousands of hectares of Kosciuszko national park.”

The project was proposed by Turnbull as a 50% expansion of the celebrated scheme, which was built over 25 years, starting in 1949. The official estimated cost of Snowy 2.0 has more than doubled and the timeline for completion has been pushed out from 2021 to 2027 since it was announced.

Asked for its response, the federal government said it would be responding to the letter in writing.

A Snowy Hydro spokesman said an environmental impact statement was being considered by the NSW Department of Planning, Industry and Environment, and the issues in the letter had been raised and responded to before. He said only 0.01% of the national park would be permanently affected by this project.

Some issues raised in the letter, including what the project would cost, how long it would take to develop and its impact on the environment, have been raised by energy analysts since the project was announced. Letter signatories to have detailed criticisms before include Woodley and Bruce Mountain, the director of the Victoria Energy Policy Centre at Victoria University.

Tony Wood, energy program director with thinktank the Grattan Institute, was not a signatory to the letter, but said he agreed with much of what was in it, including that there should be an independent review, not least because it was public money being invested.

He said the process by which Snowy Hydro had been announced before there had been a feasibility study was “ordinary to say the least”, and some of the claims about its financial case remained unclear.

“Surely they could clear the air by tabling an independent review of the project, given it’s not a typical or listed company,” he said. “The government is the shareholder on behalf of the people of Australia.”

A further concern some have raised but that is not mentioned in the letter is that the development risked giving the Snowy Hydro company, which is owned by the federal government, too great a hold over the energy market. In his book Superpower, economist Ross Garnaut suggested Snowy Hydro be broken in two so the existing generator and retailer was separate to a new body responsible for ensuring constant capacity to balance electricity supply and demand across the national grid.

The group behind the letter suggested a review could be taken by the Productivity Commission, Infrastructure Australia, the federal chief scientist, Alan Finkel, or the NSW chief scientist and engineer, Prof Hugh Durrant-Whyte.