"[2904] Pleading mere 'unjust enrichment'. A pleading that asserts in the abstract that P was unjustly enriched at D's expense will usually be struck out. The authorities require the basis of such an allegation to be explained in the pleading.
In the light of general statements in David Securities Pty Ltd v Commonwealth Bank of Australia, courts have been averse to pleadings which simply appeal to 'idiosyncratic notions of what is fair and just', or which plead generalised claims based on unjust enrichment.
...
[2908] General. The shift from implied contract to unjust enrichment as the conceptual basis of most restitutionary causes of action means that the general structure of a pleading in such a claim for restitution of money paid should be:
(1) the payment (benefit) by the plaintiff (at the plaintiff's expense);
(2) the basis for the claim, that is, the operative unjust factor (for example, mistake); and
(3) the causal link between (1) and (2).
In David Securities Pty Ltd v Commonwealth Bank of Australia the High Court rejected the proposition that a plaintiff who relies on a cause of action falling within an accepted category, such as restitution for mistaken payments, need show superadded injustice of retention or unfairness.
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[2912] Recovery of money previously paid under contract. Where the payment was equal to or less than the amount required under a contractual obligation, it is not sufficient to plead the payment under the contract and a causative unjust factor such as mistaken or duress. The ineffective nature of the contract itself must also be identified. This means that there are three elements in such claims:
(1) the payment;
(2) the ineffective nature of the contract; and
(3) the basis upon which restitution is claimed.
The first element requires no explanation. Pleading the second element requires a statement of the facts why the contract is ineffective. Reference must be made to the character of the ineffectiveness: that the contract was void when the payment was made, or discharged or rescinded after the payment was made. Of course, this requires reference to the facts relied upon that lead to the conclusion of law that the contract was ineffective. Particulars must be given. The third element looks to matters such as a total failure of the agreed return, or the presence of mistake, improper pressure, fraud or illegality, or the exercise of a judicial (usually statutory) discretion in favour of the plaintiff, such as the basis for relief against forfeiture."
[Footnotes omitted].