This study updates design rainfalls for the Brisbane River Basin, Australia, employing rainfall data from eight CMIP5 General Circulation Models (GCMs) across two Representative Concentration Pathways (RCP 4.5 and RCP 8.5) and four future timeframes (2016–2045, 2036–2065, 2056–2085, and 2075–2104), totalling 64 climate futures. Future design rainfalls were computed for Annual Exceedance Probabilities (AEPs) of 1%, 2%, 5%, and 10%, spanning durations from 1 to 7 days. For each climate future, an Annual Maximum Series was established, and a Gumbel distribution was applied to estimate precipitation for various AEPs. Comparisons were made with baseline (stationary) design rainfalls and those derived from Australian Rainfall & Runoff (AR&R) methodology based on temperature scale factors. GCM-derived design rainfalls indicate a general reduction in depths for most durations and AEPs under both RCP scenarios. Specifically, RCP 4.5 demonstrated significant reductions in design rainfalls for the 1% AEP, ranging from −9.61% (1-day duration) to −21.40% (7-day duration). Similarly, RCP 8.5 showed decreased depths, except for 1-day, 2-day, and 3-day design rainfalls in the 2075–2104 timeframe, which exhibited marginal increases. AR&R methodology yielded substantially higher design rainfalls, with increases reaching +11.6% for 1% AEP and 10% AEP. While GCM-derived rainfalls imply adequacy of current drainage infrastructure, AR&R methodology suggests a more conservative estimate, emphasizing the need for cautious climate data interpretation.