Maunsell Australia Pty Ltd have developed a new program to simulate airflow and pollution in road tunnel networks, tailored to the particular environmental influences and objectives applied to road tunnel design. The “MaunSim” software solves the dynamic network airflow problem with jet fan forcing as required by a control algorithm and vehicle behaviour being any described function of time for each tunnel section. Pollution generation is a function of grade, speed and vehicle type, currently following methods described in PIARC publications. Pressure and pollution boundary conditions can be specified at portal and additional air intakes. The behaviour of the model has been checked for the mathematically tractable cases of a step change in jet fan operation and advection of a pollution ‘sine wave’ along a tunnel. The computational method is seen to be stable for Courant numbers of less than unity. Finally, operational data from a 4km twin-tube tunnel with crossovers and ramps is being used to verify the ability of MaunSim to replicate with sufficient accuracy, tunnel airflow and pollution behaviour with the Sydney traffic fleet.