Greens leader Christine Milne says the major parties' fear of taking on the big mining companies will cost the budget $20 billion.
Senator Milne has blasted Labor and the coalition for failing to "fix" the mining tax, despite the Pre-election Economic and Fiscal Outlook (PEFO) forecasting a $30.1 billion deficit for 2013/14.
"It is no use Labor and the coalition having a going-nowhere debate on the GST when there is money to be had from fixing the mining tax right now," she told reporters in Hobart.
"Most Australians are fed up with the fact that there is almost an agreement between Liberal, Labor and the big end of town, the mining industry, to not talk about fixing the mining tax."
Senator Milne accused Prime Minister Kevin Rudd and opposition leader Tony Abbott of lacking the spine to stand up to the big miners.
"It's because the mining industry ran a major TV campaign against Kevin Rudd and it terrified the coalition and Labor," she said.
"They made a determination there and then that they would accept the fact that the big miners would not have to pay a fair share."
The PEFO reports mining tax proceeds will rise from $850 million this year to $2.5 billion in 2016/17.
Senator Milne says that's a write-down from $3.5 billion for the current year.
She said a Senate inquiry had found the tax was "flawed beyond belief" and the parliamentary budget office had costed revenues under the Greens' policy at $20 billion over the forward estimates.
Falling revenues were hurting single parents, the unemployed and students, the senator said, while $8 billion was being spent on "cruelty" to asylum seekers.
"This just shows you how the government has abandoned not only a duty of care to people but any decency," she said.
Senator Milne called on Mr Abbott to immediately release the coalition's policy costings.
She said the Greens' costings would be assessed again by the parliamentary budget office but would change little as a result of the PEFO.
