A report obtained by the ABC says coal prices will fall significantly and exports from Australia's biggest coal port will decline if Adani's Carmichael coalmine goes ahead.
If the coalmine went ahead, it would add 40 million tonnes a year to the market and global coal prices would fall by $3.80 to $65 a tonne.
Competition from the mine would reduce exports from the port of Newcastle by 11 to 12 million tonnes a year, which would lower the coal royalties NSW receives.
The report comes as the Queensland government is reportedly negotiating a "royalties holiday", followed by a phase-in of royalty payments, which could amount to $1.2 billion in revenue forgone.
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