Unreasonableness and the Shifting Language of Judicial Review

Unreasonableness is one of the three primary grounds of challenge in judicial review. In New Zealand, unreasonableness is now acquiring new purpose as a conclusory rather than primary ground of review. The finding that a decision is unreasonable is legal language to say that it is materially flawed and reviewable. It is by nature an umbrella concept that has the potential to colonise many aspects of judicial review. Any decision based on material error or defect can be categorised as unreasonable. My paper questions whether this was a considered development of the law or the legacy of loose judicial terminology. Either way, I conclude that the conclusory ground is now part of the law of judicial review in New Zealand. This development is symptomatic of the way judicial review has evolved in the courts throughout the post-war era. Even as a primary ground of review, unreasonableness was parasitic on the courts finding some other independently-reviewable error in the decision-making.