Relevant principles
50 The relevant principles in relation to legal unreasonableness were conveniently summarised in BHL19 v Minister for Immigration, Citizenship, Migrant Services and Multicultural Affairs (2020) 277 FCR 420; [2020] FCAFC 94 at [131]:
First, the concept of legal unreasonableness concerns the lawful exercise of power. Legal reasonableness, or an absence of legal unreasonableness, is an essential element in the lawfulness of decision-making.
Second, the Court's task in determining whether a decision is vitiated for legal unreasonableness is strictly supervisory. It does not involve the Court reviewing the merits of the decision under the guise of an evaluation of the decision's reasonableness, or the Court substituting its own view as to how the decision should be exercised for that of the decision maker. Nor does it involve the Court remaking the decision according to its own view of reasonableness.
Third, there are two contexts in which the concept of legal unreasonableness may be employed. The first involves a conclusion after the identification of a recognised species of jurisdictional error in the decision-making process, such as failing to have regard to a mandatory consideration, or having regard to an irrelevant consideration. The second involves an "outcome focused" conclusion without any specific jurisdictional error being identified.
Fourth, in assessing whether a particular outcome is unreasonable, it is necessary to bear in mind that, within the boundaries of power, there is an area of "decisional freedom" within which a decision maker has a genuinely free discretion. Within that area, reasonable minds might differ as to the correct decision or outcome, but any decision or outcome within that area is within the bounds of legal reasonableness. Such a decision falls within the range of possible lawful outcomes of the exercise of the power. It is only if the outcome falls outside the area of decisional freedom that it can be said to be legally unreasonable.
Fifth, in order to identify or define the width and boundaries of this area of decisional freedom and the bounds of legal reasonableness, it is necessary to construe the provisions of the statute which confer the relevant power. The task of determining whether a decision is legally reasonable or unreasonable involves the evaluation of the nature and quality of the decision by reference to the subject matter, scope, and purpose of the relevant statutory power, together with the attendant principles and values of the common law concerning reasonableness in decision-making. The evaluation is also likely to be fact-dependant and to require careful attention to the evidence.
Sixth, where reasons for the decision are available, the reasons are likely to provide the main focus for the evaluation of whether the decision is legally unreasonable. Where the reasons provide an evident and intelligible justification for the decision, it is unlikely that the decision could be considered to be legally unreasonable. However, an inference or conclusion of legal unreasonableness may be drawn even if no error in the reasons can be identified. In such a case, the Court may not be able to comprehend from the reasons how the decision was arrived at, or the justification in the reasons may not be sufficient to outweigh the inference that the decision is otherwise outside the bounds of legal reasonableness or outside the range of possible lawful outcomes.
Seventh, the evaluation of whether a decision is legally unreasonable should not be approached by way of the application of particular definitions, fixed formulae, categorisations, or verbal descriptions. The concept of legal unreasonableness is not amenable to rigidly defined categorisation or precise textural formulary. That said, the consideration of whether a decision is legally unreasonable may be assisted by reference to descriptive expressions that have been used in previous cases to describe the particular qualities of decisions that exceed the limits and boundaries of statutory power. The expressions that have been utilised in past cases include decisions which are "plainly unjust", "arbitrary", "capricious", "irrational", "lacking in evident or intelligible justification", and "obviously disproportionate": Muggeridge [v Minister for Immigration and Border Protection (2017) 255 FCR 81] at [65]. It must be emphasised, however, that the task is not an a priori definitional or "checklist" exercise: [Minister for Immigration and Border Protection v] Singh [(2014) 231 FCR 437] at [42]. Rather, it involves the Court evaluating the decision with a view to determining whether, having regard to the terms, scope, and purpose of the relevant statutory power, the decision possesses one or more of those sorts of qualities such that it falls outside the range of lawful outcomes.
Eighth, defective, illogical, or irrational reasoning or fact finding may support a finding that the ultimate decision or exercise of discretion was legally unreasonable, particularly where the illogicality relates to a critical matter upon which the decision or exercise of discretion turned…
51 The following additional points were noted in respect of arguments based on alleged illogicality or irrationality in BHL19 at [143]-[146]:
...Illogical or irrational findings made by a decision maker "on the way" to a final conclusion may establish jurisdictional error: [Minister for Immigration and Citizenship v] SZMDS [(2010) 240 CLR 611] at [132]; see also Minister for Immigration and Citizenship v SZRKT (2013) 212 FCR 99; [2013] FCA 317 at [151]-[153]; SZWCO v Minister for Immigration and Border Protection [2016] FCA 51 at [61]-[62]; Minister for Immigration and Border Protection v SZUXN (2016) 69 AAR 210; [2016] FCA 516 at [54].
It is critical to emphasise, however, that illogicality or irrationality in this context must mean something more than emphatic disagreement with the reasoning or findings: SZMDS at [124]; CQG15 v Minister for Immigration and Border Protection (2016) 253 FCR 496; [2016] FCAFC 146 at [61]. If "probative evidence can give rise to different processes of reasoning and if logical or rational or reasonable minds might differ in respect of the conclusions to be drawn from that evidence, a decision cannot be said by a reviewing court to be illogical or irrational or unreasonable, simply because one conclusion has been preferred to another possible conclusion": SZMDS at [131].
It should equally be emphasised that, for an administrative decision to be vitiated for jurisdictional error based on illogical or irrational findings of fact or reasoning, "extreme" illogicality or irrationality must generally be shown, "measured against the standard that it is not enough for the question of fact to be one on which reasonable minds may come to different conclusions": SZRKT at [148]. The "critical question" whether an administrative decision is irrational, illogical, and not based on findings or inferences of facts supported by logical grounds, "should not receive an affirmative answer that is lightly given": SZMDS at [40]. A high degree of caution must be exercised before concluding that a finding is irrational or illogical in order to ensure that the Court does not embark impermissibly on "merits review": SZMDS at [96]; SZVAP v Minister for Immigration & Border Protection (2015) 233 FCR 451; [2015] FCA 1089 at [14]-[15].
It should finally be observed, in this context, that in considering whether an administrative decision maker's decision or exercise of discretion was the product of, or was materially affected by, illogical or irrational reasoning or factual findings, the decision maker's reasons should not be the subject of over-zealous scrutiny: Minister for Immigration and Ethnic Affairs v Wu Shan Liang (1996) 185 CLR 259 at [272].