The Poatina Power Station Cavern was designed with a focus on the newly emerging rock mechanics theory and principles that were rapidly developing during the 1950’s and 60’s. This included measurement of the in-situ rock mass stress condition and photo-elastic analysis of the induced stress around the planned underground openings. These studies led to the adoption of many ‘firsts’ in rock mechanics that included a trapezoidal roof shape, installation of stress relieving slots and fully encapsulated grouted rebar bolts. Based on historical documentation of the construction of the cavern, a three- dimensional numerical model of the cavern construction sequence has been developed. The model is able to provide an accurate match to the observed and monitored ground conditions during construction that includes observed failure modes and instrumentation data. Based on the calibrated model outcomes, Hydro Tasmania was able to undertake a more informed review of the risks associated with the current and future ground support capacity and were able to reliably assess rehabilitation requirements for the cavern support system.