As with many major mountain regions of the world, Kā Tiritiri o te Moana/the Southern Alps of New Zealand act as a substantial water tower for the surrounding lowland regions. This water is heavily utilised for electricity generation and irrigated agriculture, as well as forming the basis for winter ski tourism and supporting a small number of (shrinking) mountain glaciers. Making use of the New Zealand contribution to the recently developed global Reference Observatory of Basins for INternational hydrological climate change detection (ROBIN), here we identify the presence of trends in hydrological drought and track back their drivers through the hydrological cycle: to trends (and variability) in meteorological drought, shifts in the seasonal cycle of snow accumulation and ablation, and on to the atmospheric circulation patterns that ultimately drive this variation. Results indicate the key role of changes in the strength of the prevailing westerly circulation in terms of orographic enhancement of rainfall on the western (windward) slopes of the Southern Alps, but also the magnitude of the immediate lee-side ‘spillover’ zone and subsequent rain-shadow to the east. The effects of thermal changes are also evident, particularly with respect to decreasing snow (vs rain) during winter and the resultant earlier and smaller seasonal snowmelt pulse leading to drier conditions in both the summer and autumn seasons.