The deputy prime minister Barnaby Joyce’s push for a $3bn extension of the inland rail project to Gladstone would unlock a “carbon bomb” of nine new coalmines and an estimated 150m tonnes of carbon emissions a year, environmental groups claim.

As the Nationals presented their final demands for signing up to net zero to the prime minister on Thursday, the Lock the Gate alliance said it was concerned the government was preparing to fund the “disgraceful” project being spearheaded by Joyce, saying it would be a “Trojan horse” to dig new thermal coalmines.

Last week, Joyce said the plan to take coal from Toowoomba to Gladstone could be “booked in”, telling ABC Brisbane he was focused on getting the project under way before the next election.

Nationals sources say Joyce was hopeful of securing the government’s commitment for the $3bn line extension as part of the negotiations between the coalition parties, as the Liberals seek agreement on a net zero carbon target by 2050 to take the UN climate summit in Glasgow next week.

The prime minister, Scott Morrison, is expected to respond to the Nationals’ final list of demands by Sunday, when the party will meet again to decide its final position.

While Joyce has previously hailed the $15bn, 1700km inland rail project as a “great carbon abatement” policy for taking trucks off highways, the extension of the line from Toowoomba to Gladstone is aimed at unlocking coalmines in the Surat Basin.

According to a pre-feasibility study commissioned by the government into the Gladstone rail link in 2020, there were “significant costs and marginal economic returns” from the extension.

It also stated that without the inland Gladstone link “it was considered highly unlikely that mines in the Surat Basin would progress through to operating stage.”

The largest of the proposed mines, Glencore’s Wandoan project, which would produce 22m tonnes of coal a year, has been granted all the necessary approvals for its various mining leases, but has been on hold since September 2013.

Lock the Gate Alliance Queensland spokesperson Ellie Smith said that choosing to fund the Gladstone rail link would put in place a “coal industry subsidy” that would undermine the government’s emission reduction targets.

“So, in return for getting a climate commitment on paper, the Coalition will actually be opening up a massive coal basin that, if developed, would blow our carbon targets out of the water,” Smith said.

She also said that farmers in the Wandoan area had fought the coal projects for many years due to the threat posed to underground water.

“Rail lines have major impacts on individual farmer’s properties and the viability of their businesses, and even just having that rail corridor sitting over the top of them for the last ten years has been detrimental to their business.”

According to engineering firm Aecom’s prefeasibility study on the Gladstone route that would run to Toowoomba, a “high volume scenario” would see total coal demand for the rail line peak at 60m tonnes per annum in 2030, maintained at this level until 2044 when production at some smaller mines wound down.

No coal tonnages are forecast from the basin after 2057.

“An extension to Gladstone would not be economically viable, with potential demand for the connection not sufficient to warrant the significant capital cost of the extension,” the report concluded.

But it also says that the Gladstone link “may increase the likelihood that individual mining projects proceed, and coal deposits which “otherwise may not be developed” are produced and exported.

“As such, the benefits to the coal sector estimated in this analysis are likely to be conservative.”

Using estimates from the government’s Greenhouse Accounts Factors, 60m tonnes of thermal coal would produce about 150m tonnes of carbon dioxide, based on 2.43 tonnes of CO₂ per tonne of coal burnt.

Shortly after retaking the Nationals’ leadership, Joyce announced the government would spend $10m on a business case for the Gladstone rail link, making the announcement with the outgoing Nationals MP for the seat of Flynn, Ken O’Dowd.

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Flynn is a key seat for the Coalition in Queensland, and is being targeted by the Labor party at the next election, which has preselected Gladstone mayor Matt Burnett.

The Liberal National party has preselected state MP Colin Boyce to try to hold on to the seat, which sits on a 8.7% margin.

Both candidates have spoken in favour of the rail link, saying it would boost jobs in the town and increase the output through the Port of Gladstone, which exports about 70m tonnes of coal per annum.

Joyce told parliament in August that Labor “had no intention” of funding the Gladstone rail link.

“It was never going to happen. All we have, and we still have, is that now they are saying they might do it to Gladstone. They were never going to do anything else of it. Of course they’re not going to do it to Gladstone, because they don’t believe in it. But we do,” Joyce said.