Scientists say the Narrabri coal-seam gas project is too great a risk to water supplies.

The $3.6-billion Santos project is entering the last phase of a state approvals process, with the NSW Independent Planning Commission (IPC) recently concluding a week  of public hearings into the project, in which it heard more than 400 submissions.

Dr John Williams is a former head of CSIRO's Land and Water division, a New South Wales Land and Environment Commissioner and emeritus professor and research associate at the Australian National University.

He is a founding member of the Wentworth Group of Concerned Scientists, which campaigns for sustainable development and water reform.

Dr Williams says not enough is known about fractures and faults in rock formations separating the deep coal seams, and shallow groundwater aquifers, which are provide crucial domestic and agricultural water supplies.

“I personally don't think it is worth the risk,” Dr Williams said.

“I don't think the current analysis is anywhere sufficient [enough] to give you confidence that there is low level of risk.”

Santos admitted having some uncertainties in its own scientific assessment of the project.

“There are many things where the report has been quite honest and says; ‘We don't know but we'll proceed and hope it works out’,” Dr Williams said.

“That's not good enough.

“I think the risk to our vegetation and habitat, the risk to our agricultural land and the risk to our water resources is such that I don't think we are in a position, in my view, to say there is no risk to any of that.”

NSW Department of Planning, Infrastructure and the Environment director David says independent water experts found any uncertainties could be managed with robust monitoring and regulation.

“None of our assessment has identified any potential or irreversible harm that would result from the project and in our view, the project does not trigger the precautionary principle,” Mr Kitto told the hearing.

Santos says the Narrabri gas project will supply about half of New South Wales' demand for natural gas, put downward pressure on prices, and deliver about $1.2 billion in royalties to the State Government.

Of almost 23,000 submissions made to the environmental impact assessment, 98 per cent were opposed.

The Narrabri Shire Council supports the project.

The planning commission should make a decision by September 4. If it is approved, the Federal Environment Minister will be able to give the green light.