The decarbonisation of New Zealand’s energy system will increase demand for electricity at the same time as fossil fuelled generation is phased out. Maintaining balance in the power system will become increasingly difficult as more variable generation is integrated and it is unlikely that the existing generation portfolio, with any additional generation, and demand side management will allow sufficient control. It will be necessary to increase energy storage and generation capacity. Pump Hydro Energy Storage (PHES) is the most cost effective mature energy storage technology; comprising 95% of active energy storage worldwide. PHES has relatively low carbon emissions, a high energy storage to investment ratio and long plant lifespans. However, costs and risks are project specific reflecting the range of scheme designs and dependence on geomorphology. Further, the time to develop PHES schemes is long and have environmental and other impact that are complex to assess. There is a lot to be gained from systematic evaluation of resources and optimisation of scheme designs. Here an overview of the technology, summary of previously proposed projects, and results from a search for a variety of scheme types is presented. To support resource evaluation, a GIS based evolutionary algorithm is developed and used to find a quasi-optimal upper reservoir location for a scheme using Lake Roxburgh as the lower reservoir and to identify reservoir locations for a closed loop scheme.