This thesis uses mixed methodologies to examine Australian broadsheet newspapers’ role in contributing investigative journalism to the public sphere over seven decades from 1956 to 2011. It explores print newspapers’ content to make findings about the quantity and qualitative features of investigative journalism over time. This thesis considers established theories of the media, democracy and the public sphere and finds that, in Australia, as in other developed democracies, investigative journalism has played a normative role informing the public sphere and promoting democracy by providing transparency and holding public figures to account. While investigative journalism is not exclusively the domain of broadsheet newspapers, the thesis finds they have contributed a significant sum of public interest investigative journalism to the Australian public sphere. However, print newspapers, especially broadsheets, have suffered circulation and revenue declines since the late 1980s. Print journalism has relied on advertising revenues to pay for it. In the digital age, the symbiotic relationship between journalism and advertising — at the core of the newspaper business model — has fractured. Media companies no longer have a monopoly on attracting advertisers, nor do they have a monopoly on reporting news. This thesis represents original and new research in Australia. It is the first study combining qualitative and quantitative methods to determine if the Australian public sphere has lost investigative reporting as newspapers experience economic decline. Empirical data were gathered through three content analyses, including: selected mastheads over five decades; selected online news sites; and newspaper stories from selected categories of journalism’s peer-reviewed Walkley awards since their inception in 1956. This study also included qualitative analysis through 22 semi-structured interviews with editors, media proprietors, investigative journalists, media analysts, academics and media sector unionists. It directly compared the contributions of print investigative journalism between broadsheets and tabloids; and more broadly examined the contributions from non-print media. The content analysis data of news websites tested whether the nascent online sphere was originating, rather than merely distributing, Australian investigative journalism. This research resulted in the acquisition of a comprehensive repository of Australian award-winning and investigative journalism from 1956 to 2011 — the first of its kind. To perform the analyses, this author derived an original operational definition of investigative journalism informed by both academic literature and media professionals. Finally, this thesis concludes what effects economic and technological changes have had on broadsheet investigative journalism, and discusses emerging trends in investigative reporting. The research contributes original findings to the scholarly literature about the state of the relationship between print investigative journalism and democratic accountability in Australia.