On appeal from a decision of the director, the Administrative Appeals Tribunal must—
(a) treat the decision as if it had been made by the chief executive; and
(b) if it proposes to vary the decision under review—vary it in a way that it could be varied under this Act; and
(c) if it proposes to make a decision in substitution for the decision of the director—make a decision that could be made by the chief executive under this Act.