Australian prime minister Julia Gillard is under pressure from the opposition and the Greens to reveal the numbers behind the government's revised mining tax, amid suggestions the revenues will fall far short of the original estimate.
A 30% tax will be levied on coal and iron ore miners.
To secure the deal, Ms Gillard scrapped a 40% tax rate originally proposed for the entire resource sector.
The federal government says the compromise will reduce expected revenues by $A1.5 billion to a total $A10.5 billion in the first two years of the tax.
But revenue in the second year will be $A2.5 billion less than originally intended.
Opposition Leader Tony Abbott says there is something "inherently fishy about the figures" and the "only way to clear it up" was for the modelling to be released.
The ABC reports the Greens want the federal Treasury to make public exactly how much the mining tax backdown will cost the government's bottom line.