This paper explores the multimodal mobility impact of implementing BRT as an open system with mixed services and transfers or as a closed system with separate BRT services. In large multimodal public transport networks, interchanges or transfers for passengers are inevitable. A zone-based fare system ensures there is no financial penalty for interchange thus minimising the interchange penalty. In Brisbane, South East Queensland, Australia, the zone-based fare system does not penalise transfers within the same zone but charges a full fare for an inter-zone transfer within a single journey. The open system BRT in Brisbane, Australia is used as a case study with a sensitivity analysis on transfer rates to understand how open system operation has facilitated multimodal mobility between conventional bus and BRT services. Web-based simulation maps compare a closed BRT scenario with the open BRT system in operation. The coverage and volume of passengers are significantly reduced in the closed system scenario, especially for routes inbound to the Central Business District (CBD), since far fewer passengers in suburban areas would have direct BRT services. In Brisbane's current open system, there are fewer opportunities for multimodal trips because buses travel on and off the BRT infrastructure. This analysis can help urban transport agencies to optimise their BRT operations and provide better public transport services.