DOUBTS have emerged over whether the giant $50 billion Gorgon gas project can safely bury its greenhouse gas emissions deep beneath the Barrow Island nature reserve off Western Australia, despite the Prime Minister, Kevin Rudd, backing the plan.

Technical experts working with Gorgon's developers, Chevron, Shell and ExxonMobil, found it was possible that carbon dioxide could leak from faults in the geological formation under the island which is supposed to act as the burial site. Their findings were disclosed in the developers' response to complaints from the Conservation Council of WA, which argued the plan to bury the gases was highly risky.

The environmental review produced by the Gorgon partners notes a ''significant area of difference'' between it and the expert panel over the the risk of a leak.

But the partners said they accepted the panel's assessment that leakage was possible and would consider how to reduce this likelihood as part of their continuing technical assessment.

Mr Rudd and the WA Premier, Colin Barnett, cleared the way for the project last week by agreeing that taxpayers would accept any long-term liability arising from a failure of the experimental burial plan. Mr Rudd said the project would be ''the world's largest demonstration'' of technology to capture and bury greenhouse gases from an energy project.

But the Gorgon project has been opposed by environmentalists and the WA Environmental Protection Authority, who say Barrow Island is home to 24 species that exist nowhere else. Of particular concern is the flatback marine turtle, which breeds on the island.

The federal Environment Minister, Peter Garrett, is due to release his assessment this week on whether the companies' plans to protect the turtle and other migratory species are sufficient to allow the project to go ahead under federal law.

A WWF spokeswoman, Gilly Llewellyn, yesterday called on Mr Garrett and all involved to consider ''an alternative safer location'' for the huge project, which includes a liquefied natural gas processing site and port.

The expert panel considered that two fissures beneath Barrow Island, known as the Pluto and Godwit faults, were possible sites where stored gas could escape to the surface. The companies said they were looking at ways to reduce the risk of leakage.

In a statement to the Herald last night, a Gorgon spokeswoman said the rock under the island offered the ''greatest containment certainty'' of any site within reach of the gas fields. She said technical assessments indicated the formation would in effect trap the carbon dioxide, ''making leakage unlikely''.

The companies' plan to bury the huge carbon dioxide releases is one of the key reasons they proposed using Barrow Island for the development, despite environmental concerns.

The EPA said in its final report on the project that if the carbon burial plan proved technically impracticable and did not go ahead ''the decision to permit gas processing on Barrow Island nature reserve should be reconsidered''.