Simulation models are an important tool to predict how farming practices influence utilisation and loss of nitrogen (N). However, many simulation exercises lack sufficient validation of N dynamics from both soil and fertiliser sources and rely on single or a few measurable N pools, potentially shifting bias from one pool to another. This study evaluated the capacity of Agricultural Production Systems sIMulator (APSIM) to simulate N fertiliser budgets in dryland sorghum and wheat systems of eastern Australia using an extensive 15N dataset from 18 field trials with up to four N rates. Key metrics included 15N fertiliser recoveries at harvest in soil and plant, and 15N fertiliser losses. Fertiliser N in APSIM was calculated as the balance between simulated N in the non-fertilised control and the respective applied N rate, and compared to relevant 15N data. APSIM's performance for fertiliser N budget simulation was tested with (i) conventional calibration fitting plant N uptake and (ii) improved calibration including the denitrification and mineralisation parameters to fit fertiliser N loss. Simulations with conventional calibration showed good agreement with phenology, soil water and plant N uptake data but largely underestimated the measured fertiliser N loss (RMSE: 23 kg N ha-1, PBIAS: - 75%). The simulated fertiliser N loss with improved calibration agreed better with the observation (RMSE: 18 kg N ha-1, PBIAS: -34%). The identified inconsistencies in the default model's representation of N cycling highlight opportunities for combined efforts of experimental research and model development to improve the reliability and utility of the model.