The Gillard Government has been untrue in its account of the effect of the revamped resources tax on forward revenue, the Coalition says.

According to Senator George Brandis, Treasury boss Ken Henry told a senate hearing yesterday the cost of the revisions to tax earnings was $4.5 billion, not $1.5 billion as declared by the Government.

"So there has been a misstatement of a magnitude of three times by the government of the effect of these changes," said Senator Brandis.

"That is what Dr Henry said, the cost of the revenue is $4.5 billion on his assessment, the Government said the cost of the revenue is $1.5 billion."

Small Business Minister Craig Emerson, meanwhile, said there would be abundant revenue in forward estimates even with the tax restructure.

An agreement with the mining sector last week saw the government slash $1.1 billion for exploration, axe refundability on mine losses and begin capital base depreciation over 25 years, not five years.

"You put all those together and you still get robust revenue over the forward years and beyond," Mr Emerson said.

Commodity price assumptions by Treasury had also been increased and are predicted to prop up revenue.