Labor's pain over its under-performing minerals resource rent tax is set to increase when the Greens reveal plans for a wide-ranging inquiry into its design and performance.
The Greens leader Christine Milne will open a new line of attack on Labor's left flank on Tuesday, using a lunch-time address to the National Press Club in Canberra to push for a full-scale Senate inquiry.
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Shadow Treasurer Joe Hockey says the Coalition would be inclined to support the Greens' proposal to hold a Senate inquiry into the Labor's minerals resources rent tax, but Tony Abbott seems less convinced.
It would need the backing of the Opposition in the Senate to be successful.
''We would be inclined to [support it] but we need to see the terms of reference,'' Mr Hockey told Fairfax Media.
The Opposition Leader, Mr Abbott, said the mining tax ''should be scrapped not reviewed'', when quizzed about it at a news conference in Ramsgate, south of Sydney on Tuesday morning.
Mr Hockey said he thought the mining tax was worth investigating because ''everyone should have a clear indication on how not to develop a tax''.
''I think we should properly investigate the differences between Kevin Rudd and Julia Gillard and Wayne Swan on the development of the tax,'' Mr Hockey said.
But the Shadow Treasurer did acknowledge that ''no amount of inquiry will fix it [the MRRT]'' and that all an inquiry would do is ''prove the tax is flawed''.
''We stand by our commitment to abolish the tax,'' Mr Hockey said.
Shadow assistant treasurer Mathias Cormann said the mining tax should be scrapped however if the Greens want another inquiry into the tax "then we won't oppose it".
"Two previous Senate mining tax inquires which I chaired clearly exposed that the mining tax is a complex mess," Senator Cormann said.
Under the Greens plan, the committee will be given the task of examining all aspects of the MRRT's design and administration including its highly troubled negotiation phase with the big three miners, and ''the extent to which the design of the tax as opposed to other factors such as commodity prices are responsible for the mismatch between actual revenue and revenue projections''.
Also in the Greens' sights are the lack of community consultation; options to strengthen the tax to raise more money from miners; its current impact on the budget; and, the extent to which its shortfall has compromised other programs.
The potentially embarrassing election-year move will put the minor party on a collision course with the party it backed to form government in the House of Representatives.
It comes as Labor struggles to explain why a super-profits tax that was projected to raise $2 billion in its first year to fund tax breaks for businesses and the low paid – itself a downward revision on previous projections – has turned in just $126 million in its first six months of operation.
Branding Labor as "too scared" to take on powerful vested interests, Senator Milne will argue the eventual tax was merely a political fix to silence the miners in the lead-up to the 2010 election. ''If a government is too scared to take on vested interests, then it is the people who can't afford ads in the paper or television, who can't afford expensive lobbyists to walk the halls of Parliament House, people who can least afford it – people like single mothers – who end up paying,'' she will say according to speech notes obtained by Fairfax Media. ''Well, the Greens aren't afraid.''
It is unclear whether the Opposition will back the move but the Greens believe Coalition support is likely, if only to embarrass the government before the election.
The Senate inquiry is separate from smaller inquiries already under way in the Senate regarding loopholes in the tax which allow big miners to depreciate assets from current market value, and which also allow state governments to jack up royalties at the expense of the Commonwealth.
Ms Milne will say she has raised concerns about the mining tax ''in public and in private with the Prime Minister and the Treasurer on numerous occasions''.
''It is now clear that they, like Tony Abbott and Warren Truss are just not prepared to put themselves on the line for the national interest,'' she will say.
''Let us put to rest the nonsense that the mining industry cannot afford to pay more.
''In 2011, the big three companies that negotiated the mining tax – BHP Billiton, Rio Tinto and Xstrata – made a combined annual profit of $27 billion.''