In any system-of-systems the potential exists for interactions between systems to occur that may affect the security of the overall system. We present a scenario that aims to highlight such problems, in particular that of security at a network boundary. This scenario considers cooperation and interactions between organisations and systems in the context of a major public event, such as a sporting or entertainment event. Based on this we present a modelling tool able to highlight potential access violations that might occur through transfer of data between multiple organisations and suggest ways to mitigate these vulnerabilities. The use of the modelling tool for network boundary checking is demonstrated, using the example scenario. Suggestions are made as to how security and effectiveness can be achieved by applying safeguards to vulnerable areas, while allowing the free flow of data between organisations where this is known to be safe.