Treasury Secretary Ken Henry has assured that the Australian government's national budget will not be at risk because of the lower revenue estimates since the landmark deal on the proposed tax on resources was finalised last week.

In a Senate hearing, Secretary Ken Henry said in Canberra today that though the Minerals Resource Rent Tax will generate lower earnings than former Resource Super Profits Tax, it would not put pressure on the budget.

"I don't think I would agree that the budget is exposed to a higher risk," Henry told a Senate committee hearing adding that estimates showed that "there is even a return to surplus in 2012-13."

Henry said the Treasury revised up its forecasts for commodity prices since the May budget and since the RSPT was announced on May 2. Commodity prices are "volatile," he added.

Prime Minister Julia Gillard's agreement extends the current Petroleum Resource Rent Tax to all onshore and offshore petroleum and gas projects.

The levy will apply once a project's return on investment exceeds 7 percentage points more than the 10-year government bond yield, currently about 5 percent. Previously it would have kicked in for any return above the bond rate.

BHP Billiton Ltd., Rio Tinto Group and Xstrata Plc approved the revised tax plan after meeting with Gillard, Treasurer Wayne Swan and Resources Minister Martin Ferguson last week.