The basis of the equitable jurisdiction to set aside an alienation of property on the grounds of undue influence is the prevention of the unconscientious use of any special capacity in or opportunity for the disponee to affect the disponor's will or freedom of judgment in reference to the transaction: Johnson v Buttress [1936] HCA 41; (1936) 56 CLR 113, 134.
The jurisdiction to set aside a transaction procured by undue influence is exercised on two bases. The first is where undue influence is proved as a fact. The second is where undue influence is presumed by reason of the antecedent relationship between the parties, and the presumption has not been rebutted: Johnson v Buttress (119); Union Fidelity Trustee Co of Australia Ltd v Gibson [1971] VicRp 69; [1971] VR 573, 575. The former is 'actual' undue influence and the latter is 'presumed' undue influence: Powell v Powell [2002] WASC 105 [120] - [121].
Actual undue influence requires proof that the transaction was the outcome of such an actual influence over the mind of the disponor that it cannot be considered to be the free act of the disponor: Johnson v Buttress (134). The source of power to practise such influence or domination over the disponor may not arise from an antecedent relationship, but may arise in the particular situation, or by the deliberate contrivance of the disponee: Johnson v Buttress (134).
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The doctrine of undue influence looks to the quality of the consent, or assent, of the weaker party: Commercial Bank of Australia v Amadio (474); Bridgewater v Leahy [1998] HCA 66; (1998) 194 CLR 457, 478.
Where a third party, who is a volunteer, obtains the benefit of a transaction tainted by the undue influence of the influencer, the volunteer will be affected by the equity (unless the volunteer has taken through some intermediate bona fide purchaser for value without notice): Bainbrigge v Browne [1881] UKLawRpCh 148; (1881) 18 Ch D 188, 196 - 197; Giarrantano v Smith (1985) NSW ConvR 55-267.
Even where the third party has given value, or provided consideration, the transaction may be voidable by the party the subject of the undue influence. Such cases often involve a guarantee or charge given by the person the subject of the undue influence to a bank to secure the indebtedness of the bank's customer.
The receipt and retention of a benefit by a third party bank arising from the customer's undue influence has generally been held to be unconscientious on three principal bases. The first is where it has participated with notice, the second is agency, and the third involves special principles historically developed and applied to married women.
As to the first of those, the third party is affected in equity where it participates in the transaction with actual or constructive notice of the circumstances giving rise to the impropriety, ie of the actual undue influence exercised, or of the circumstances from which the presumption of undue influence arises: Yerkey v Jones (677); Bank of New South Wales v Rogers [1941] HCA 9; (1941) 65 CLR 42, 55, 70 - 72, 85 - 86; Commercial Bank of Australia v Amadio (464); Garcia v National Australia Bank (408); Budget Nominees Pty Ltd v Registrar of Titles (1988) V ConvR 54-311, 63-988. (But see also [204] below.)