This paper outlines a methodology employed to apply Ventsim ventilation modelling software to gas drainage and nitrogen injection systems. Use of the software to account for compressible flow, specified gas compositions and the resistance of conduits of various dimensions makes it suitable for both ventilation circuits and gas reticulation systems. Unlike mine ventilation circuits, in gas drainage systems it is necessary to specify flow rates arising from gas pre drainage holes as these are dependent on gas reservoir properties. This requires a mass flow balance to be obtained between underground gas drainage holes and that reporting to surface pump(s). In nitrogen injection systems it is convenient to create an interface between the gas drainage reticulation system and the mine workings which in turn is connected to the ventilation system. However, it is also possible to keep the two systems separate for simplicity and tuning. The results of applying these strategies to an operating gas pre drainage system are described. These provide the “as built” pipe friction factors obtained by matching the model results to observed differential pressures. The Pike River re-entry nitrogen injection system has also been modelled. This uses the same principles as the negative pressure gas drainage models but with high positive pressures of up to 1,000 kPa (10bar).