The budget surplus heralded by the federal government for fiscal 2013 may not be realised after all following reports that the targeted $7.4 billion revenue from the minerals resource rent tax could suffer considerable shortfalls.

According to a story ran by The Australian Financial Review, the MRRT might only manage to collect more than $4 billion by 2012 as prevailing commodity output and prices run in counter with projections earlier made by Federal Treasurer Wayne Swan.

The Coalition said on Thursday that Australia is now facing some billion-dollar budget black hole because of the wrong assumptions pushed forward by the government of Prime Minister Julia Gillard, which initially predicted that up to $12 billion in revenues could be amassed once the mining tax takes effect.

That was reduced to just over $7 billion when Ms Gillard struck a deal with major resources companies prior to seeking a fresh mandate in the August national elections.

Opposition resources spokesman Ian Macfarlane scored the government for its misleading revenue figures that only managed to erode the credibility of federal authorities' economic competence.

Macfarlane said that if the report was to be believed, then the Gillard government is simply reduced to "inflicting enormous uncertainty on the energy and resources sector because of its poorly designed tax, which further disintegrates every day."

Also, opposition leader Tony Abbott expressed alarm that the MRRT seems to appear problematic each day as he told ABC that Ms Gillard must now contend with the spectre of brewing confrontations with either mining firms or the country's premiers.

Both ways, Abbott claimed, are damaging to the proposed mining tax and should leave a gaping hole on MRRT's aimed revenues.

On his part, opposition assistant treasury spokesman Mathias Cormann blamed Swan for hiding the true figures that surround the economic impact of the mining tax, telling The Australian that the deputy prime minister reneged on his role as prime economic manager by not divulging the whole story.

Senator Cormann decried Swan's dodging tactics on senate initiatives to inquire on the models underpinning the revenue assumptions of the MRRT, acts that he said led to imminent impacts that threaten "the budget bottom line and it could put the government's illusionary surplus at risk."

Macfarlane said since new developments are proving that Ms Gillard's handling of the mining tax only inflicts uncertainty on the energy and resources industry, the government should all-together scrap the MRRT's eventual implementation.