The post-closure biosphere assessment and modelling in support of the general licence application for a deep geological repository for radioactive waste provides the basis for evaluating radiological consequences should radionuclides released from the geological repository reach the surface and near-surface environments. The assessment approach comprises the modelling of radionuclide migration in the biosphere following potential releases from the geosphere to a local aquifer over timescales of tens of thousands to more than a million years. A stylised approach is adopted, including reference and alternative biosphere conditions, consistent with both national and international regulations and guidelines. The model is implemented in Nagra’s SwiBAC code, which allows radionuclide fluxes to the biosphere to be translated into potential doses to humans. This process yields equilibrium biosphere dose conversion factors (BDCFs) for each radionuclide for use in the analysis of radiological consequences, which occurs in a separate assessment step. Reference biosphere calculations are based on present-day conditions in Northern Switzerland, represented by a valley setting with a temperate climate. These conditions support a wide range of potential radionuclide transport and exposure pathways, including agricultural water use, soil-plant transfer, and local food consumption. Three alternative biospheres are also assessed to address uncertainties in long-term climate and geomorphology, including (i) a warmer-drier climate, where aquifer and surface flows are reduced while irrigation is increased, (ii) a cold climate with permafrost, where the exchange between soil and the aquifer is reduced, and (iii) a drained farmland setting with a high water table. BDCFs vary significantly between these cases, increasing by up to about an order of magnitude in warmer-drier conditions and decreasing by up to about five orders of magnitude under cold-climate conditions. These results highlight the key role of the shallow aquifer and transfer pathways within the valley agriculture system. Deterministic and probabilistic sensitivity analyses further highlight the influence of sorption in the shallow aquifer, hydrological fluxes, and agricultural transfer pathways. Together, the reference and alternative cases constrain key uncertainties and provide a robust, conservative basis for evaluating long-term radiological safety.