Economic modelling by the well-known resource analytics firm Wood Mackenzie found new coal supply from new mines in the Galilee Basin would reduce the price of coal by 25 per cent and cut NSW coal exports by about 80 million tonnes a year.

A separate report by the Australia Institute think tank has used that modelling to estimate the impact on the NSW budget from lost mining royalties.

"Based on Wood Mackenzie's analysis, the development of the Galilee Basin will reduce the value of NSW coal royalties by $10.2 billion to 2035," it concludes.

The Wood Mackenzie modelling was commissioned by The Infrastructure Fund which owns a 50 per cent stake in the world's largest coal port at Newcastle. It compares a "no Galilee Basin coal" scenario to the impact of 150 million tonnes of Galilee Basin coal production being put on the world market.

So far the NSW government has downplayed the potential impact on NSW from Adani's giant Carmichael mine and other Galilee Basin projects.

In a statement to the Herald a spokesperson for the NSW Minister for Resources, Don Harwin, said: "NSW is operating in a global coal export industry and produces high quality coal that is exported mainly to Japan, Korea, China and Taiwan. The Division of Resources and Geosciences advises that as the Adani project is planned to export to India, the project won't affect NSW exports as our state has minimal exports to India."

But the view that NSW mines producing high quality coal will not be affected by new supply from the Galilee Basin is contradicted by Wood Mackenzie's analysis. 

It shows price changes in one grade of coal are almost immediately passed on to other grades.

Adani has sought a loan of nearly a billion dollars from the federal government's Northern Australia Infrastructure Facility to construct a rail link from the Galilee Basin to the Adani-owned coal port at Abbot Point.

But Jonathan van Rooyen, General Manager of Investments at The Infrastructure Fund, said subsidies for new coal mines in North Queensland will have "devastating consequences" for existing coal regions like the Hunter Valley.

"The [Wood Mackenzie] modelling shows that subsidising the Galilee coal mines will reduce coal production in the Hunter Valley by more than 85 million tonnes, that's a reduction of more than a third," he said. "That means a lot less jobs, a lot less investment and a lot less royalty revenue for the NSW Treasury."

Mr van Rooyen called on the NSW Premier, Gladys Berejiklian, to "stand up for the coal miners and businesses of the Hunter Valley and stand up for the taxpayers of NSW who will miss out on billions of dollars in royalties if the Adani and other Galilee mines go ahead."

Last month's state budget forecast the NSW government to collect more than $6.2 billion from mining royalties over the next four years.

The Australia Institute's report found that under Wood Mackenzie's Galilee Basin projects' "go ahead" scenario the mining royalties collected by the NSW government will be almost $1.2 billion per year lower by 2035 than under a "no Galilee" scenario.

"It is basic economics that a large new subsidised coal supply will push down prices and push unsubsidised mines out of production," said the Australia Institute report's author Rod Campbell.

"The NSW government and NSW coal industry should strongly oppose the development of the Galilee Basin, particularly with subsidy from federal and Queensland state governments."

Mr van Rooyen said he found it "hard to fathom" why the federal government would consider using taxpayers' money to support the development of the Galilee Basin.

"Hopefully the modelling makes clear to the Federal Government, and the Board of the [North Australia Infrastructure Facility], that the idea of subsidising the construction of a massive new coal basin at a time of flat world demand is bad for the rest of the Australian coal industry."

The Wood Mackenzie modelling showed existing coal producing regions of in Queensland, including the Bowen Basin, would also be hard hit by new supply from the Galilee Basin.

The modelling did not take into account the possibility of future change in climate change policies.