This study contributes to the debate on the effectiveness of mandatory environmental reporting regulation in the Australian context. Our longitudinal in-depth quantitative and qualitative analysis evaluates the changes in compliance with Australian mandatory environmental reporting regulation (the Corporations Act, s. 299(1)(f)) of Australian listed companies, as well as their changes in reporting quantity and quality from 1997 to 2017. We measure disclosure quality from a multidimensional and innovative perspective: the comprehensiveness, negativity, and substance of the disclosure. Adopting the institutional perspective of legitimacy theory and the various dynamics of the legitimacy framework developed by Suchman (1995), as well as a substantive legitimation perspective, we extend the interpretive power of legitimacy theory in the area of mandatory environmental reporting. The results show that, overall, mandatory environmental reporting in Australia is improving with respect to increased compliance with s. 299(1)(f), and increased quantity and quality of reporting. Our research provides an in-depth examination of the process by which mandatory environmental reporting regulation becomes influential. Our findings show that s. 299(1)(f) remains effective 20 years after its adoption.