The Gillard government's decision to pacify mining companies by revamping the resource tax could cost the budget $35 billion in revenue this decade, a report says.

In defence of its ''breakthrough agreement,'' brokered with global miners BHP Billiton, Rio Tinto and Xstrata, the government says the revisions would raise $1.5 billion less than the resource super-profits tax (RSPT) in its first two years.

However, a report by Goldman Sachs JBWere shows these figures undervalue the revenue foregone under the new levy.

It found the revised regime would raise $3.2 billion lower than former prime minister Kevin Rudd's RSPT in its first two years from 2012, more than double the Gillard government's approximate.

The difference between the two tax recommendations is then predicted to increase over the decade as mining companies lift production and profits.

By 2020, budget revenue is expected to be $35 billion lower than it would have been under the original tax proposal modelled by Treasury secretary Ken Henry.

The findings of Goldman analyst Hamish Tadgell suggest that although the new minerals resource rent tax (MRRT) would collect $20.9 billion in revenue this decade, the RSPT would have raised more than $56 billion over the same period.

Forecasts for the tax beyond 2013-14 have not been released.

The long-range projection follows Dr Henry's disclosing this week that the government had depended on higher commodity price assumptions when it predicted the revamped mining tax would raise $1.5 billion less than its forerunner.

The report presupposed that commodity prices would be well below current near-record levels after 2014.