Hyperpolarised xenon-129 (129Xe) production via continuous-flow spin-exchange optical pumping often produces lower than predicted 129Xe polarisation. Frequently, thermodynamics within polariser systems is not considered, as well as spatially variant changes in rubidium (Rb) source distributions and changes in 129Xe relaxation over time. This work models realistic Rb and temperature distributions within a large-scale 129Xe-Rb polariser cell. Results show that Rb density varies significantly depending upon incident photon flux and Rb source distribution in the cell. Modelling allows estimation of the Rb presaturator length required at different gas flow rates in order to reach optimal Rb density and high 129Xe polarisation.