Recent years have witnessed a substantial increase in reporting on social and environmental (SE) issues by major corporations (Gray, 2006). A recent trend indicates that organisations have moved on from the terms ‘social and environmental reporting’, to the more vague term, ‘sustainability’ reporting (Adams and Larrinaga-González, 2007). Most prior research into social and environmental (SE) reporting has examined disclosures in corporate annual reports (Parker, 2005), but there has been relatively little research into public sector agencies’ reporting of these matters. Also, until recently, there has been little interest in ascertaining the views of preparers of accounts regarding SE and intellectual capital reporting (Unerman and Guthrie, 2007). This study inquired into the reasons for sustainability reporting (SR) within a group of Australian public sector ‘best practice’ organisations. The study’s main aim was to analyse ‘why’ the SE reporting had been undertaken, considered from a preparer’s perspective. The research method used involved semi-structured interviews with key preparers in the various organisations. Among the findings of the study is that the public sector agencies examined aspects of ‘sustainability’ and their reports have been informed by the latest GRI (and the sector supplement for public agencies). From a preparers’ perspective, disclosure of sustainability information in the annual report was for the purpose of informing stakeholders, but mainly internal ones. However, the annual report was considered to be one of a number of media for reporting the complexities of an organisation’s sustainability objectives and activities. Finally, the study also investigated the reasons why the public sector organisations started the journey of voluntary SE reporting. The findings show that generally one key individual within the organisation pioneered and championed the process of SR. Also, in most cases, the organisations started with the triple bottom line (TBL) or balanced scorecard (BSC) and had only recently moved to the GRI framework, the reason for this being that it was considered to have an international reputation and standing, which gave it a certain legitimacy.