Moomba was discovered in 1966 and is the largest known gas field in the Cooper Basin. The Permian Toolachee Formation reservoir Unit 'C' in the Moomba Field South Dome contains most of the established gas reserves, within a closure area of approximately 125 square miles upon which 35 wells have been drilled to June 1978. This government-authorised report sets out to elucidate the stratigraphy and re-assess the proved and probable original gas-in-place reserves for Unit 'C', following on from the original Sydney Gas Market Joint Venture (SGM) Reserves Study of August 1975. The revised estimate of 2P OGIP for the subject reservoir when compared to the SGM findings results in a major reduction in reserves. In support of this conclusion: -- field wide stratigraphic subunits within the Toolachee Formation are correlated; -- drill stem test results are interpreted to reveal that some sands are independent of others and have their own bases of pay; -- log analyses support the interpretation of drill stem test results; -- bases of pay as a function of porosity and permeability have been defined for the identified stratigraphic subunits. A brief comparison is made with the results of the SGM Reserve Study. The two studies are comparable in that reservoir engineering data have not been altered from those used in the SGM. Recommendations are made concerning the conduct of future field development in the Cooper Basin.