"Accordingly, it is not legitimate to determine
whether an enrichment is unjust by reference to
some subjective evaluation of what is fair or
unconscionable. Instead, recovery depends upon the
existence of a qualifying or vitiating factor such
as mistake, duress or illegality. As this Court
stated in Westpac Banking Corporation (1988) 164
CLR 662 at p.673:
'In other words, receipt of a payment
which has been made under a fundamental
mistake is one of the categories of
case in which the facts give rise to a
prima facie obligation to make
restitution, in the sense of
compensation for the benefit of unjust
enrichment, to the person who has
sustained the countervailing
detriment.'
As La Forest J stated in Air Canada v. British
Columbia (1989) 59 DLR (4th) 161, at p 192, the
two species of mistake (i.e., fact and law) should
be 'considered as factors which can make an
enrichment at the plaintiff's expense "unjust" or
"unjustified"'.
The respondent's submission that the appellants
must independently prove 'unjustness' over and
above the mistake cannot therefore be sustained.
The fact that the payment has been caused by a
mistake is sufficient to give rise to a prima facie
obligation on the part of the respondent to make
restitution. Before that prima facie liability is
displaced, the respondent must point to
circumstances which the law recognizes would make
an order for restitution unjust (Westpac Banking
Corporation (supra)). There can be no restitution
in such circumstances because the law will not
provide for recovery except when the enrichment is
unjust. It follows that the recipient of a
payment, which is sought to be recovered on the
ground of unjust enrichment, is entitled to raise
by way of answer any matter or circumstance which
shows that his or her receipt (or retention) of the
payment is not unjust."