You should post your legal advice, or at the minimum the name of...

You should post your legal advice, or at the minimum the name of the person providing you with such advice, because without an attribution all you have posted is unsubstantiated opinion.

Mark Robinson SC, a suitably qualified expert on the subject, presented a legal paper on the subject of judicial review.

Here are some extracts which IMO have pertinance:-

As to improper purpose, this ground of judicial review is best explained by the descriptionof it in s 5(1)(e) read with s 5(2)(c) of the Administrative Decisions (Judicial Review) Act1977 (Cth) which provides: “The making of the decision was an improper exercise of thepower conferred by the enactment in pursuance of which it was purported to be made [in thatthere was] an exercise of a power for a purpose other than a purpose for which the power isconferred.” The common law position in Australia is that the improper purpose complainedof must be or have been a substantial purpose in the sense that the decision or act complainedof would not have occurred but for the improper purpose: Thompson v Randwick MunicipalCouncil (1950) 81 CLR 87 at 105–106 and Warringah Shire Council v Pittwater ProvisionalCouncil (1992) 26 NSWLR 491. Improper purpose is also sometimes linked to Wednesburyor legal unreasonableness – see for example, East Melbourne Group v Minister for Planning[2008] VSCA 217 at [340]–[341] (per Ashley and Redlich JJA).

As to dictation - this ground of judicial review applies when a decision-maker is possessedof personal statutory decision-making power. In that circumstance, the decision-maker mustnot be dictated to by politicians or more senior public servants, or by anyone else. Further,blind adherence to government policy might well provide evidence of dictation. A decisionmakermust not abdicate his or her personal judgment or personal duty to anyone. Theleading cases are: R v Anderson; Ex parte Ipec-Air Pty Ltd (1965) 113 CLR 177 and AnsettTransport Industries (Operations) Pty Ltd v Commonwealth (1977) 139 CLR 54.

As to irrelevant/relevant considerations - A decision-maker must take into account onlyrelevant considerations and must not take into account irrelevant considerations. The leadingcase in Australia is Minister for Aboriginal Affairs v Peko-Wallsend Ltd (1986) 162 CLR 24at 39–42 where Mason J (as he then was) set out the position.

As to legal unreasonableness, this ground was first identified by the High Court of Australiain Minister for Immigration and Citizenship v Li (2013) 87 ALJR 618; 297 ALR 225. TheCourt held that every statutory discretionary power has attached to it by the common law arequirement that it be exercised reasonably, having regard to the statutory purpose of thepower. In this regard, the High Court held that the decision-maker must not be “unreasonablein a legal sense” (at [72]).This new notion of “legal unreasonableness” (as it is described in Li at [66]) does not involvethe courts in undertaking a review of the merits of an exercise of discretionary power (ibid).The courts must look to the scope and purpose of the statute conferring the discretionary power and divine its real object (at [67]) applying ordinary statutory construction principles,such as in Project Blue Sky Inc v Australian Broadcasting Authority (1998) 194 CLR 355.

The ground of legal unreasonableness reflects the requirement of the law that a decisionmakermust understand his or her statutory powers and obligations (at [71]) and it will beestablished as a jurisdictional error when, for example:
1. where no sensible authority acting with due appreciation of its responsibilities wouldhave so decided (at [71]);
2. the decision-maker has failed to give adequate weight to a relevant factor of greatimportance (at [72]);
3. the decision-maker has given excessive weight to an irrelevant factor of noimportance (ibid);
4. reasoned illogically or irrationally (ibid);
5. when the decision is a disproportionate response by reference to the scope of thepower (at [73]-[74]);
6. when a decision lacks evident and intelligible justification (at [76]);
7. where it is not apparent how a conclusion was reached, but the decision itselfbespeaks error (at [82] and [85]).

Legal unreasonableness is an inference that is to be drawn from the facts and from the mattersfalling for consideration in the exercise of the statutory power (at [76]).