Radon was used to quantify groundwater inputs into Australian ICOLLs.
Groundwater discharge varied seasonally and spatially.
Groundwater discharge ranged from 2.6 ± 1.7 to 7.3 ± 8.9 cm d−1.
90% of the water inputs to the ICOLLs was via groundwater discharge.
ICOLLs are groundwater dominated coastal systems.