Prime Minister Rudd and WA Premier Barnett have signed off on a more than $3 billion carbon risk for Australian taxpayers under the Gorgon gas deal, the Australian Greens said today.

Part of the agreement with developers Chevron, ExxonMobil and Shell indemnifies them against liability for leakage of any of the carbon they store, passing that multi-billion dollar risk onto the taxpayers of Western Australia and the Commonwealth.

The project is expected to attempt to sequester 130 million tonnes of CO2. At a carbon price of $25 a tonne, expected in the early years of any emissions trading scheme, leakage of that carbon would entail a carbon cost of $3.25 billion on Australian taxpayers.

"Thanks to Mr Rudd and Mr Barnet's deal, Australian taxpayers will bear the full risk for a massive carbon experiment by some of the world's biggest polluters," Australian Greens Deputy Leader, Senator Christine Milne, said.

"This is a critical lesson to remember on the day when the Government and Opposition joined forces to exempt polluters from the renewable energy target.

"Geosequestration of carbon dioxide is unproven. Now, if the carbon leaks, not only will the community bear the full cost to the climate, but we will also have to pay the carbon cost of that leakage!"

Senator Milne also expressed concern at the conclusion of this deal before the environmental impact assessment of the project has been completed.

"What Martin Ferguson and the Government have done is once again wiped their boots on the Minister for the Environment."