This chapter examines the fourth estate principle of the ‘public interest’ as informing important reforms to defamation law in Australia. It offers an historical and international context to the law around investigative and public interest journalism and tracks some of the financial and legal hurdles investigative journalists and their publishers face when defending defamatory material in their reports. Shortcomings of the key defences to defamation are highlighted with case examples. It then details an important recent defamation law reform in Australia – the introduction of a new ‘public interest’ defence modelled on a 2013 UK predecessor – and considers the extent to which it might benefit the enterprise of investigative journalism using historical and international comparisons. The chapter concludes by speculating how the new defence might have applied hypothetically to a current high-profile trial as a means of examining its potential utility for public interest journalism.