The general law in Australia is that in the absence of any statutory provision money paid under a mistake of law (as opposed to a mistake of fact) is irrecoverable. There are however a number of ’exceptions’ to the rule that are particularly relevant when discussing exactions by public officials. If the payer was acting under duress or any form of compulsion or, coercion or undue influence, those factors vitiate the payment and the payment is therefore recoverable regardless of the mistake.
"The Present Situation in Australia Regarding Recovery of Payments Made Pursuant to Ultra Vires Demands,"
Bond Law Review:
2, Article 6.
Available at: http://epublications.bond.edu.au/blr/vol4/iss2/6