Coastal lagoons face anthropogenic pressures worldwide, and understanding groundwater discharge to these sensitive environments can enable better management of water budgets and nutrient inputs. The main objective of the study was to quantify groundwater seepage into a large and shallow coastal lagoon using a detailed radon (222Rn, a natural groundwater tracer) mass balance. To assess and reduce uncertainty, three wind-speed scenarios were modelled in two seasons using two 222Rn-in-groundwater endmembers. Groundwater discharge to the lagoon ranged between 5.2 ± 5.8 m3/s to 18.7 ± 19.6 m3/s during summer and 0.9 ± 2.2 m3/s to 8.1 ± 10.5 m3/s during winter. Wind-driven radon evasion was the most influential component of the radon mass balance. Higher summer wind speeds resulted in groundwater discharge estimates 3.5 times greater than those during winter. We carried out an in-depth uncertainty analysis of the radon mass balance results, which confirmed the importance of accounting for individual uncertainties, particularly the most sensitive parameters of the model. Overall, results from the radon mass balance revealed groundwater discharge to the lagoon was 1-2 orders of magnitude greater than previous localised seepage meter estimates. This is likely because radon tracer approaches quantify both one-directional discharge and bi-directional porewater exchange at a larger spatial scale than traditional seepage meter methods. In spite of large uncertainties, the possible range of reasonable estimates revealed groundwater seepage as an important component of the water budget at this site. This study highlights the benefits of broad-scale tracer-based mass balances for quantifying groundwater discharge to coastal lagoons and the value of in-depth uncertainty analysis to increase confidence in seepage flux estimates.