Environmental radiocarbon was used to study aspects of groundwater origin, transport and mixing in the aquifer system underlying northern Victoria. The clearing of native vegetation and the advent of irrigation on the northern plains have upset the hydrological balance, caused water tables to rise and in some areas caused the waterlogging and salination of the land. Groundwater shallower than 30 m only yielded modern radiometric dates. Groundwater in the ‘Calivil/Renmark Aquifer’ in the Loddon and Goulburn Valleys has yielded ages up to 20 000 years and has a flow rate between 0.13 and 0.01 m day‐1. These values agree with the mean pore velocity calculated using Darcy's Law.