Lake Onslow in New Zealand’s Central Otago region is under consideration for a major expansion to become the upper reservoir of a significant pumped storage scheme with 5 TWh energy storage capacity. The increased evaporation loss would likely lead to a requirement for compensation input of new water to the lake to maintain its mean outflow to the Teviot River. A simple expression is obtained that gives an upper bound of 1.5 m3s-1 for compensation flow if the lake is expanded to 60 km2. The compensation water could be derived from the pumping operation at the expense of some energy loss. Alternatively, water input without energy loss might be achieved by diverting some flow into Lake Onslow from the nearby Taieri River.