Q18: Are you acting for a proper purpose?

Is your purpose clearly articulated and appropriate, or could it be alleged that you are motivated by an improper purpose?

If there are multiple purposes for your decision, which is dominant?

A decision can be invalidated where an improper purpose materially influences the outcome, even if a proper purpose also exists.  However, a decision made for a proper purpose is not invalid because it has an ancillary purpose, as long as that ancillary purpose is also within the ambit of the Act.

A State Services Commission decision in 1985 to relocate a staff member pursuant to an administrative transfer provision was overturned.  The decision was held to be influenced by conduct that could have been subject to disciplinary action (the staff member had been expressing views seen as political and inconsistent with their role).  The relevant statute provided a disciplinary process that should have been used instead. The existence of the disciplinary issue and conduct were held to be material to the decision to use the administrative transfer provision, and therefore it was overturned.
Poananga v State Services Commission [1985] 2 NZLR 385
The Court of Appeal confirmed that the Department of Conservation could use buildings on a reserve, not just for the purposes of that reserve, but to administer other reserves as well.
Attorney-General v Ireland [2002] 2 NZLR 220

One way of asking this question is, ‘but for that [improper] purpose, would I still take this course of action?’  Why is the decision being made?  Is each purpose material to the decision being made?

The Court of Appeal rejected an argument that Ministers could not use the procedure in the State-Owned Enterprises Act 1986 to direct changes to the content of Timberland’s Statement of Corporate Intent, with the effect of preventing West Coast beech forest harvesting. The power could be used in that manner and the Court held that it is not for the courts to prevent the Crown exercising legitimate power.
Lumber Specialists Ltd v Hodgson [2000] 2 NZLR 347
Decisions by the Minister for Canterbury Earthquake Recovery to direct changes to regional planning documents were set aside by the High Court.  The Court found that, alongside legitimate statutory purposes, the Minister had acted to resolve broader urban boundary development issues and resolve ‘reverse sensitivity’ issues relating to noise at the airport.  These were not relevant purposes for the Minister to act under the statute.
Independent Fisheries Ltd v Minister for Canterbury Earthquake Recovery [2012] NZHC 1810
The Supreme Court held that revocation of the protected status of conservation land under section 18(7) of the Conservation Act 1987 could only occur where protection was no longer warranted by the intrinsic conservation values of the specific land protected. A calculation based on “net gain” due to land transfers was not permitted.
Hawkes Bay Regional Investment Co Ltd v Royal Forest and Bird Protection Society [2017] NZSC 106