Widespread (~200,000 ha) drought-induced dieback of Eucalyptus viminalis, the dominant species in the Tablelands Snow Gum, Black Sallee, Candlebark and Ribbon Gum Grassy Woodland threatened ecological community, is severely impacting native vegetation health and causing habitat loss in the Monaro. Past debate about the causes of E viminalis mortality in this area (Ross and Brack, 2015; 2017; Jurskis, 2016) had equivocal findings. There is international consensus that greater understanding of the links between tree mortality and climate change, when trees are under chronic or acute water stress, is needed (Allen et al. 2010, Booth 2017, Correa et al. 2017, Prober et al. 2017a, 2017b, Curtis 2019). An understanding of hydrogeological landscape (HGL) processes (Moore et al. 2018) in this landscape, specifically access to groundwater, allowed causal factors for tree dieback to be evaluated. Extreme climate impacts since the early 2000s have influenced root access to groundwater in parts of the landscape (this study) in a manner that caused stress for an extended period afterward (see White 1969, Lynch and Cowood 2018) making trees susceptible to secondary impacts (e.g. insect strike with limited insect predation due to land clearing/open landscapes and changes in fire cycles) resulting in localised tree mortality. This impact is apparent even when predisposing cohort factors (e.g. tree age, natural range) are considered. Clarification of the HGL processes operating in areas of tree dieback, explains why the pattern of impact differs in different parts of the landscape allowing strategic targeting of restoration actions. For the 2060-2079 NARCLIM (12 model) climate projection, E viminalis shows high (>80%) consensus for climate stress across the Monaro (MacKenzie, 2018; personal communication). Use of the HGL framework allows the evaluation of other potential stressors (e.g. land/water salinity, solute/toxin transport, soil sodicity/erosion susceptibility) and informs targeted natural resource management to accommodate future climate and land-use change.