The jury is still out on whether coal seam gas (CSG) or coal is the biggest villain, Greens Leader Bob Brown says.
Federal politicians have been debating whether farmers should have the right to block miners from extracting (CSG) from their land.
Senator Brown on Wednesday questioned whether this so-called "transition" fuel is actually any cleaner than coal.
"The presumption that the damage done from gas is half that done by coal is under very, very serious questioning," he told reporters in Canberra.
"The jury is out on it because the information base is still very shaky."
Senator Brown said more work needed to be done to determine "which of the fossil fuels is the biggest villain".
Policy makers were missing "a proper estimation of the leakage of methane out of the whole coal seam gas process" from test drilling through to export, he said.
Greens climate change spokeswoman Christine Milne on Tuesday asked the government whether CSG extracted in Australia was less greenhouse gas intensive than black coal "on a life cycle analysis basis".
In response, Labor frontbencher Penny Wong told the Senate she wasn't sure what studies had been undertaken but "gas is generally lower in emissions than coal".
The Greens want Australia to move straight to renewables with gas used only as an interim measure "and not as a long-term investment".
Senator Brown on Wednesday backed independent MP Tony Windsor's push for the federal government to be given greater powers to veto CSG projects if they have the potential to damage water resources.
The Tasmanian senator said Mr Windsor's bill to protect aquifers would "complement" his own move to give farmers greater powers to block exploration and extraction on their land.
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