The mining sector gained $7.5bn in the Gillard government's remodelled tax, according to new figures that unveiled Treasury significantly undervalued earnings from the resource super-profits tax (RSPT).

The government admitted yesterday it miscalculated its original tax, which could have earned double the announced $12bn in its first two years, based on the latest commodity price projections.

Wayne Swan released updated budget estimates yesterday, taking in Treasury's new approximates for commodity prices that would boost the budget profits by $4.9bn over the next four years.

While the government's earnings have been raised by the extra mining income, the updated budget suggests Australia's expansion is slacking and is now predicted to reach only 3 per cent this financial year, a decline from a budget forecast of 3.5 per cent.

The budget is still projected to go back to surplus in 2012-13, although by a margin of $3bn, instead of the $1bn forecast when it was announced in May.

The revised data end two weeks of avoiding by the government on the generosity of its gifts to the mining sector since Julia Gillard discarded the RSPT in favour of the mineral resources rent tax (MRRT).

At the time, the new prime minister said the earnings under the MRRT would decline only $1.5bn to $10.5bn, even with disounting the headline rate from 40 per cent to an effective rate of 22.5 per cent and doubling the threshold at which it kicked in.

Treasury did not explain if its new forecast was based on a significant upgrade to prices of key commodities, particularly iron ore and coal, which increased mining company profits and the commonwealth's tax profits. Without the $6bn lift to mining tax receipts coming from the higher prices, the MRRT would have collected only $4.5bn or $7.5bn less than the levy championed by former Prime Minister Kevin Rudd.

The Treasurer confessed yesterday that had the higher prices been implemented to the RSPT, it could have earned up to $24bn.