Inclusiveness is critical to the ability of political organisations, especially social movement organisations (SMOs), to accommodate different social and political groups' interests. A higher level of inclusiveness helps organisations to mobilise more effectively and generate greater momentum in creating social and political changes at a much faster pace. Not only that, when SMOs are less inclusive, they become more vulnerable to external attacks and infighting. These negative conditions also threaten an organisation's existence. Nevertheless, it is a perennial challenge for different political organisations to establish and maintain higher inclusiveness to resolve collective action problems, for instance climate change in Australia. Thereby, this thesis seeks to answer the following question: how can different types of social movement organisations (SMOs) establish and maintain higher inclusiveness? To answer this question, this thesis also examines the ways different organisational mechanisms operate in two groups with very different levels of institutionalisation, Australian Conservation Foundation (ACF) and Extinction Rebellion Australia (XR Aus), but finds that institutionalisation, while important, is not necessarily the decisive factor. [...]