Realistic values of specific storage (Ss) for groundwater systems are important to determine the spatial extent and timing of changes in groundwater heads when the groundwater system is mechanically stressed. However, numerical groundwater models of underground excavations typically assume a constant literature value of Ss. Our research used both in situ and laboratory obtained data. High frequency pore pressure data was evaluated to understand the variability and changes in Ss within sedimentary strata overlying a longwall coal mine. Pore pressure data from a vertical series of 6 vibrating wire piezometers (50 to ~278 m depth) recording at hourly intervals used barometric pressure loading for the period before and during mining. The site was located near the centre of a longwall panel that extracted coal at a depth of 300-400 m. The data was processed to calculate loading efficiency and Ss values by multi-method analyses of barometric and earth tide responses. In situ Ss results varied over one to two orders of magnitude and indicated that Ss was changing before and after extraction of underlying coal seams. Laboratory geomechanical tests were undertaken to compare with in situ findings. The vertical leakage of groundwater within the constrained zone (~10 to ~150 m depth) was found to be limited before mining, although some degree of vertical hydraulic connectivity was observed. Depressurization was evident in the fractured zone directly overlying the coal seam, and Ss changes at ~250 m depth indicated this confined aquifer may have became unconfined, while other zones remained confined. Our results demonstrate that high frequency pore pressure data can provide realistic time series Ss values. We have shown, for the first time, that variability of Ss can be significant, and that these changes can provide important insights into how shallow and deep groundwater systems respond to underground mining.