This paper maps networks of affiliation and interest in the Australian Twittersphere and explores their structural relationships to a range of issues-based ad hoc publics (Bruns, Burgess 2011). Using custom network crawling technology, we have conducted a snowball crawl of Twitter accounts operated by Australian users to identify more than one million users and their follower / followee relationships, and have mapped their interconnections. In itself, the map provides an overview of the major clusters of densely interlinked users, largely centred on shared topics of interest (from politics through parenting to arts and sport) and/or socio-demographic factors (geographic origins, age groups). Our map of the Twittersphere is the first of its kind for the Australian part of the global Twitter network, and also provides a first independent and scholarly estimation of the size of the total Australian Twitter population. In combination with our investigation of participation patterns in specific thematic hashtags, the map also enables us to examine which areas of the underlying follower / followee network are activated in the discussion of specific current topics – allowing new insights into the extent to which particular topics and issues are of interest to specialized niches or to the Australian public more broadly. Finally, we investigate the circulation of links to the articles published by a number of major Australian news organisations across the network.