The validity of an administrative act done under one statutory power but supportable under an alternative power depends on statutory interpretation of the particular regime, not a general rule. The critical question, adapting Project Blue Sky, is whether it was a purpose of the legislation that an act done in reliance on an inappropriate statutory power, but supportable under another, should be invalid. A material difference in the consequences of the alternative powers is always a relevant consideration but is not necessarily determinative. The broad proposition in Saatchi & Saatchi that it would 'generally be wrong' to uphold an act under an unstated power should not be followed. Separately, a restatement of a statutory eligibility formula does not constitute 'reasons' for a determination, but advertising requirements that are subsequent notification provisions rather than essential preliminaries will not invalidate a determination if breached.
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