The Australian Greens may block welfare changes in the budget they say are too punitive as well as cuts to heritage and environment projects.
Greens leader Bob Brown said the budget was "lacklustre" and his party, which will hold the balance of power in the Senate from July 1, would discuss changes with the treasurer.
"We will be working with the government to reverse the very petty cuts that we see forced on this obsession with a cut across the board when more money is flowing into the country," Senator Brown said.
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"We guarantee supply and we guarantee that we can improve this budget and we will be strong on both those things."
Senator Brown said the measures to get young parents and disabled people off welfare and into work were "harsh and punitive".
"We will be looking at those to see whether they are essential," he said.
The Greens are concerned the boost to skilled migration may come at a cost to Australian workers and that cuts to spending on the National Gallery and National Library do not reflect a modern-thinking nation.
They will also take issue with a $500 million cut in renewable energy programs and stress the need for greater public transport spending.
Senator Brown said funding could be found by reversing the corporate tax cut for big firms to start in 2013 and reverting to the early version of the mining profits tax.
Asked why he thought Treasurer Wayne Swan did not mention the words environment or indigenous in his speech, Senator Brown said: "I just think there's a mind-blank there."
