Accurate numerical modelling packages are now widely used to model and design semiconductor optoelectronic devices. Existing packages have difficulties in modelling current flow through vertical cavity surface emitting laser (VCSEL) structures, because of the large number of interfaces (~100) between layers of different composition in the VCSEL mirror stacks. The energy band discontinuities at the interfaces between the different layers lead to numerical instabilities in conventional packages. We have developed a software package based on an alternative, fast, numerical technique, which can treat the large number of interfaces in VCSEL structures and provide drift-diffusion I-V-T characteristics for realistic layer structures. The simulation package we have developed uses the drift-diffusion model to calculate the current-voltage characteristics, as a function of temperature, for semiconductor structures containing many layers (~50). The package therefore provides a stable and efficient route to calculate VCSEL I-V characteristics. It was developed specifically for red VCSELs (operating at ~650 nm), and so includes material parameters appropriate to InGaAlP active regions, with (Al)GaAs/Al(Ga)As Bragg mirror stacks.