Bunching usually occurs when faster trucks and slower trucks are mixed in a truck-shovel mining system. This paper presents the development of discrete-event simulation model to estimate the impacts of bunching on the productivity and efficiency of a truck-shovel system. The bunching effect on production, BEP, is defined to estimate the production sensitivity to the bunching effect on the truck fleet. The simulation results show that the mixed truck fleets with varying performance can cause significant bunching effect if the hauling trucks are from multiple loading sites or dumps. Depending on whether the fleet is over-trucked or under-trucked, the bunching has significant impact on both the fleet productivity and the equipment utilisation. Furthermore, the fleet with higher BEP takes a priority of overtaking the fleet with lower BEP to increase the productivity.