The Birch Creek catchment in central Victoria is covered predominantly by Pliocene – Quaternary Newer Volcanics basalt lavas with scattered eruption points; the lavas overlie Tertiary valley-fill sands and Palaeozoic bedrock. Groundwater within the catchment can be divided into 3 types. Type 1 groundwater is fresh (<600 uS/cm, < 60 mg/L Cl) and occurs close to eruption points where recharge is rapid; type 2 is more saline and its EC (900-1500 uS/cm), which is strongly correlated with the Cl concentration (60-280 mg/L), increases downgradient. This reflects the progressive input down the flow path of recharge that has been impacted by evapotranspiration during infiltration through the low permeability clay-rich soils on the basalt. Type 3 groundwater is found in 3 bores located within 1 km of each other, and has quite different chemistry, with relatively high salinity (900-1500 uS/cm) but low Cl levels (<50 mg/L), so the high salinity is not due to evapotranspiration, in contrast to Type 2 groundwater. Type 3 groundwater also has high PCO2 (10-0.6 to 10-1.8) and enriched del13C (>-10), similar to the naturally effervescent Hepburn mineral springs located ~15 km east of Birch Creek. The Hepburn spring water contains high levels of magmatic CO2, presumably derived from an underground reservoir emplaced during eruption of the Newer Volcanics, and type 3 groundwater has a component of CO2 from the same source. The input of CO2 made type 3 groundwater more acidic, increasing the weathering of olivine and plagioclase in the basalt and releasing substantial amounts of Mg2+ and Na2+ into this groundwater, accounting for its relatively high salinity but low Cl concentration. Therefore, magmatic CO2 is more widespread in central Victoria than previously realised, and its distinctive hydrochemical signature (high salinity but low Cl) can be used to identify its presence in groundwater throughout Victoria.