A public authority’s decision may be challenged on the grounds of unreasonableness (sometimes described as ‘irrationality’). The test that the Court will apply most often today to determine whether a decision is reasonable is whether the decision is “within the range of reasonable responses” that a decision-maker could have had in the circumstances. However, there is no one standard test that will be applied in all cases to determine a decision’s reasonableness. Instead, the test to be applied and the intensity or closeness of the Court’s review of a decision’s reasonableness will vary according to the subject-matter and the nature and gravity of what is at stake.
Cases that may be subject to low intensity review include those involving political judgment, e.g. matters of national economic policy. This means that the threshold for showing unreasonableness may be very high (e.g. where a decision must be shown to be arbitrary or perverse, manifestly without reasonable justification, devoid of any plausible justification, or one where there is an error of reasoning which robs the decision of logic).
However, decisions involving a person’s rights or constitutional principles (e.g. rights that are protected by the rule of law – liberty, accessibility of the law, certainty of the law, accountability, access to justice) are usually scrutinised more intensely or closely. In these cases, a lower threshold of unreasonableness is applied, and the Court may require substantial evidence that the decision goes no further than necessary to achieve the decision’s objective, and that objective must be sufficiently important. This is very similar to the considerations the Court apply under the proportionality test.