Many Australian underground coal mines have or are likely to encounter areas of increased gas content, which are difficult to drain. A number of factors have the potential to impact the overall efficiency and effectiveness of gas drainage from the mined coal seam. A mine based investigation was undertaken at an operating coal mine working in the gassy Bulli seam of Australia’s Illawarra coal measures. Gas production data from 279 inseam gas drainage boreholes was evaluated relative to a variety of coal properties and mine operational factors to determine the impact of each on gas production performance. Whilst the design of the boreholes and the drainage time had some impact on gas production it was the coal properties that had the most impact. Coal rank, ash content, gas content, seam thickness and gas composition were all found to impact gas production. In particular, total gas in place and degree of saturation had the most significant impact on coal seam gas production performance. Various recommendations are suggested.