A Santos plan to develop coal seam gas in northern NSW has been found by an independent expert panel to lack key data that potentially underestimates impacts on threatened ecosystems and ground and surface water.

The report by the Independent Expert Scientific Committee, released without fanfare last month, identified "knowledge gaps, uncertainties and data limitations" within the Environmental Impact Statement submitted by the energy company for the $1 billion-plus Narrabri project.

Among those gaps is the question of just how much wastewater the planned 850 CSG wells will extract and the wider impact on groundwater. The experts had "low confidence in the water balance modelling" provided by Santos.

"Contingency actions are not provided to address the possibility of excess water storage if other beneficial reuse options [such as for irrigation] are not available," the panel's report said.