If you're helping the kids with their homework and they forget to carry the one or something you can generally show them how they'll get their sums a bit wrong.

But you'd reckon they'd have to be trying hard to get a $2Billion apparent deviation wouldn't you?   For most of us the billions columnn stands out a bit.

Treasury has conceded that the mining tax looks unlikely to bring it the $2 billion it was forecast to, increasing the pressure on Treasurer Wayne Swan as he tries to find savings before the budget.

Mr Swan's department has told a parliamentary inquiry into the tax that ''while it is difficult to draw conclusions'' nine months into the financial year, mining tax collections to date suggest revenue for 2012-13 will be lower than estimated last October.

Last month, it was revealed that the mining tax had made just $126 million in its first six months, despite government forecasts of $2 billion for the entire year.

Mr Swan has blamed the fall in coal and iron ore prices and the high Australian dollar. Last month he noted there had been a partial recovery in profits: ''We will have to see how prices go over the next six months.''

The mining tax will be put under the microscope this week, as the Senate's economics committee tries to get to the bottom of why the tax has underperformed. Treasury and the Australian Taxation Office - which are examining the reasons for the ''apparent deviation'' between forecast and revenue - will appear before the committee on Wednesday in Canberra.