Reasonableness is a central, defining concept in Australian administrative law. All justiciable aspects of administration – determinations of fact, questions of law, discretion, and delegated legislation – are subject to judicial review for unreasonableness. Other grounds of judicial review similarly apply across the board. However, the courts most explicitly have to navigate the boundaries between the ‘legality’ and the ‘merits’ of administrative action when applying the reasonableness grounds. Theoretically, the legality/merits dichotomy lies at the heart of Australian administrative law doctrine, defining the respective roles of administrative agencies, courts, merits review tribunals, and Ombudsmen. The courts' use of unreasonableness as a ground of judicial review tells us where the boundary between the two actually lies in practice.

This chapter provides an overview of the role of reasonableness, and the related concepts of rationality and proportionality, in judicial review of administrative action. It begins by discussing why the law requires administrators to act reasonably, then the relationship between the concepts of reasonableness, rationality and proportionality. It then considers how the courts use these concepts when reviewing findings of fact, exercises of discretion and delegated legislation. It concludes with a defence of the role of reasonableness as a ground of judicial review, against the charge that it is too indeterminate to provide a useful standard of good administrative decision-making.

Why is reasonableness legally required?

Much has been written on why the courts can hold unreasonable administrative action invalid.