‘Improper purpose’ is a recognised ground, both at common law and under statute, which, if established, can enable the courts to invalidate the exercise of a discretionary power. At common law, this ‘elementary proposition’ was first established in England ‘in cases concerning the exercise of powers of compulsory acquisition’. At the federal level in Australia, under the Administrative Decisions (Judicial Review Act) Act 1977, s5(1)(e) and s6(1)(e) provide respectively for judicial review of a decision or conduct engaged in for the purpose of making a decision on the ground ‘that the making of the decision was an improper exercise of the power conferred by the enactment in pursuance of which it was purported to be made’. ‘An improper exercise of a power’ referred in relation to a decision or conduct engaged in the making of a decision is defined in both instances as meaning ‘an exercise of power for a purpose other than a purpose for which the power is conferred’. This fundamental principle was articulated in the following broad fashion by Gibbs CJ in The Queen v Toohey; Ex parte Northern Land Council (hereafter ‘Toohey’):