Illegality is divided into two categories: those that, if proved, mean that the public authority was not empowered to take action or make the decision it did; and those that relate to whether the authority exercised its discretion properly. Grounds within the first category are simple ultra vires and errors as to precedent facts; while errors of law on the face of the record, making decisions on the basis of insufficient evidence or errors of material facts, taking into account irrelevant considerations or failing to take into account relevant ones, making decisions for improper purposes, fettering of discretion, and failing to fulfil substantive legitimate expectations are grounds within the second category.
