River flow regime is important for ecosystem integrity and biodiversity, and is impacted by artificial water exploitation and regulation, extreme events and climate change. Environmental flows are designed to mitigate these impacts. In this work, we used catchment hydrological output simulated by the Soil & Water Assessment Tool model, evaporation rates calculated with the General Lake Model and a water resource management tool to calculate a water balance for a sub-tropical reservoir located in southeast Queensland, Australia under extreme dry conditions impacted by climate change. Two environmental flows, the 90th percentile of the daily flow duration curve (Q90) and the 90th percentile monthly flow for each season (Q90M), were used to assess water availability for multiple purposes including environmental releases and water supply from a subtropical water supply reservoir. Risks for the downstream river system were assessed using a flow alteration screening method by comparing a 20-year baseline period of no-dam from 1990 to 2009 with future climate change and water regulation conditions with the dam in place. A significant decrease in maximum flood peak, persistence of high-flow duration and the annual discharge from the reservoir occurred with future climate change and water demand scenarios compared with the no-dam baseline. The future changes in flow regime indicate a medium-to-high ecological risk for high flow conditions and moderate ecological risk during low flow conditions compared with the no-dam condition. An increased water demand in combination with future climate change leads to high ecological risks occurring by the end of this century under the high emission scenario of representative concentration pathway 8.5, regardless of the water regulations and environment flow management. The findings of this work indicate moderate to high inability to meet water demand under future climate-induced hydrological change. Therefore, catchment managers will need to take further actions to mitigate impacts from future climate change and population growth on water resources in this subtropical system.