Despite good rainfall, the groundwater resources on Kangaroo Island are limited, with localised good quality groundwater available only in small supplies. Just over 75% of all wells drilled on the island have a salinity of over 2000 mg/L, with 95% of all wells yielding less than 1 L/sec. This is mainly because of the unfavourable geology. The deep clayey weathering zone of the widespread Cambrian Kanmantoo Group basement rocks, a relic of tropical palaeosol formation in Tertiary times, has restricted infiltration recharge to and throughflow in those fractured rock aquifers, and has contributed to high salinities of resident groundwater. Elsewhere, consolidated Bridgewater Formation dune aeolianite deposits may contain small lenses of low salinity groundwater, but these lenses have the potential to be contaminated by underlying saline groundwater if overpumped. Groundwater supply potential exists in the Cygnet River area from Recent alluvium and underlying Tertiary limestone, plus the Quaternary aeolianite along the south coast. Small supplies may also be available from unconsolidated sand dunes. Field investigations (drilling and geophysical surveys) will be necessary to confirm this potential, but possible benefits arising from this work will be localised. Any development requiring a consistent groundwater supply rate of over 3 to 5 L/sec would require desalination of brackish or saline groundwater, if such yields are available.