Environment Protection Minister Peter Garrett has written to both companies asking for extra information on water management strategies amid concerns by Queensland's Coordinator-General and local communities about the effect on the Great Artesian Basin, one of the few reliable sources of fresh water in inland Australia.

Mr Garrett has given Santos and BG until October 11 to submit revised environmental impact statements, potentially pushing out both companies' final investment decision.

The letter follows a site trip to Gladstone and Curtis Island by Mr Garrett more than a week ago and meetings with both BG and Santos about their projects, where he expressed concerns about giving the projects the environmental green light.

The delay on a decision means it is unlikely voters will know the fate of thousands of potential jobs in Queensland until after the election.

Treasurer Wayne Swan last night moved to reiterate the government's support for the $50 billion liquefied natural gas industry in Queensland, a battleground state that Labor needs to win.

"It is well known that the Minister for Resources and I are two of the biggest supporters of the LNG "We are acutely aware of just how important this industry is for jobs, growth and the community — and recognise the need to get this resolved quickly.

"These are large and complex projects and, whether we like it or not, we need to operate within the confines of the rules," Mr Swan said.

While BG and Santos say they still expect to make their final investment decisions this year, it is a setback, particularly for BG, which was the closest to a decision on its $20 billion project.

A spokesman for Mr Garrett said the extra time was needed to assess properly "areas identified as deficient in the Queensland Coordinator-General's report".