Recognition of the adverse technical implications of known active aquifer pressure support and interpool communication between Permian gas reservoirs of the southeastern Cooper Basin requires that an estimate of aquifer size be undertaken to complement material balance OGIP determinations. This will allow prediction of the effects on gas pool recovery factors of water encroachment and formation water production from existing wells, thereby quantifying and identifying the potential for reserve losses occasioned via depletion of non-producing reservoirs from offset production. This report, which covers a study area encompassing the Toolachee and Nappacoongee-Murteree Blocks where the majority of gas fields display aquifer pressure support, details the method used to define the aquifer systems, quantifies each aquifer's size, and calculates the volumes of water and gas in each of seven mappable reservoir system packages.