Senator Bob Brown on ABC television last night pledged to put the revised tax through a ''very close Senate scrutiny'' with a purpose of studying the repercussions on small business.

According to the Greens leader, the revamped resource profits tax will remove $4 billion out of the annual budget each year over the next 10 years, which is almost thrice the $1.5 billion flagged by Deputy Prime Minister Wayne Swan when he declared the revision last week.

The senator's comments come after Treasury Secretary Ken Henry confirmed yesterday that the government presupposed an increase in commodities prices and export volumes in flagging only a $1.5 billion loss in tax receipts due to the revised tax. Original government approximates of the tax indicated it would accumulate $12 billion more than the $10.5 billion in predicted tax receipts under the revised impost.

Prime Minister Julia Gillard reduced the tax rate for the proposed resource profits tax to 30 per cent last week from the original 40 per cent championed by former Prime Minister Kevin Rudd.

Under Ms Gillard, the minerals resource rent tax slashes the count of affected companies from 2500to 320. The MRRT also narrowed the types of commodities to be levied to iron and coal.

The debatable tax on the mining sector, first declared two months ago, stoke a bitter multi-million dollar public relations war against Mr Rudd, which ousted him from leadership in the Labor Party last month.