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• The first study to characterize the multidimensional behaviors of unregulated floods. • A strong geographic cohesion identified in the multidimensional behaviors of floods. • The geographic cohesion partly explained by the relations between floods and ENSO. • The decreases in floods in southern Australia projected to continue in warmer future. Changes in peak magnitude, volume, frequency and duration of floods obtained from a peak-over-threshold sampling in 780 unregulated catchments show significant differences between northern and southern Australia over 1975–2012. Increases of the flood properties are mainly located in northern Australia, while decreases are mostly in southern Australia. These changes could be dominated by inter-annual and/or decadal variability of floods. The multidimensional behaviors of flood change across Australia can be described by three distinct groups (i.e. no changes, increases and decreases in all flood properties), showing strong geographic cohesion. The geographical consistency between the changing patterns of flood properties and spatial patterns of vapor transport anomalies during the El Niño-Southern Oscillation (ENSO) positive phase could partly explain the geographic cohesion of flood changes. In a warmer future, the observed decreases in floods in southern Australia are projected to continue with high model agreement, while only magnitude and volume of floods in northern Australia are projected to increase but with high uncertainties. The diametric changes in flood magnitude between northern and southern Australia are projected to be more evident in extreme (i.e. 50-year) floods than small (i.e. 5- and 20-year) floods. [ABSTRACT FROM AUTHOR]