This article seeks to investigate the way in which social media can affect terrorist events. Using the 2014 Sydney siege as its primary focus, it will argue that the public’s social media activity, particularly the capacity to engage in ‘reporting’ of live events as they occur, can shift the tactical advantage from counterterrorism officials to the perpetrator. Situational awareness theory will be used to analyse how the public’s Twitter activity during the event had the capacity to enhance the perpetrator’s decision-making and therefore his overall capacity to execute the attack. The article will analyse the Martin Place Siege Joint Commonwealth—New South Wales Review, particularly, Chapter 10, Public Communication. The Review had shortcomings in terms of its failure to fully analyse the role of social media during the Sydney siege and the way in which it impacted upon events. The article therefore seeks to highlight the need for law enforcement and government agencies to take into account developments within social media, which have added a new dimension to terrorist activity. Failure to take account of these developments will diminish the capacity of law enforcement and government to respond effectively to similar events in the future.