Greens senator Scott Ludlum says Labor's plan to bring forward the start date for an emissions trading scheme (ETS) is a terrible idea.
He says his party might not support it if it comes before parliament.
Defence Minister Stephen Smith on Monday confirmed the federal government wanted to shift to a floating price from July 2014, a year before the fixed price period was due to end.
He denied the move was an admission that the government's carbon tax had been poorly designed, adding Labor had wanted to introduce an ETS earlier but was blocked by the Greens and the coalition.
Treasurer Chris Bowen says, either through the parliament or the election, Labor will be seeking a mandate to make the change.
The Rudd government would require the support of the Greens or the coalition in the Senate to pass any changes to its carbon pricing laws.
Senator Ludlum says the Greens will wait and see what the legislation looks like, but they aren't in the mood to support anything that cost billions and further delays real action on climate change.
"It's a terrible idea," he told ABC's Q&A program on Monday.
"I think we would be very unlikely to support something that costs us $4 billion and another 12 month delay."
Mr Smith said an announcement to explain how the government would find the budget savings to make this change was just "hours and days away".
Deputy Opposition Leader Julie Bishop said the whole plan was a Labor "con", but had to fend off claims Tony Abbott had used the issue to stir up scepticism about climate science.
Earlier when criticising Labor's ETS plan, Mr Abbott described emissions trading as a "so-called market" dealing in an "invisible substance".
"Well.. it's (carbon dioxide) not visible to the naked eye, so he's making an evident point," she said.
"The point is, he was talking about the fact that emissions trading schemes have not been a success anywhere in the world."
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