Equitable Estoppel The application of estoppel at common law and equity to the exercise of statutory power is a topic itself deserving of a substantial paper.68
A number of species of estoppel have been identified as having an application in public law. These include estoppel by representation which comprises common law estoppel, relating to present facts, and equitable or promissory estoppel relating to the future. Issue of estoppel and proprietary estoppel also have potential application.69
It is well established that a public authority cannot be required, by the application of doctrines of estoppel, to exceed its statutory powers or breach its statutory duties. That would involve equity amending the statute. That is not to say that a statutory power or duty might not, in appropriate circumstances, be capable, on general principles, of a construction accommodating obligations arising from equitable principles. But such a construction would by definition allow the performance of the obligation intra vires or in accordance with the relevant statutory duty.
There may be put to one side the classes of case in which officials or public authorities enter the realm of private law by making contracts, acquiring or disposing of property or engaging in tortious conduct. There the private law, including equity, applies to them. This was well exemplified in Verwayen v The Commonwealth70 where the Commonwealth was held estopped in negligence litigation from invoking a limitation period which it had previously indicated it would not invoke. It is increasingly a feature of modern life that statutory authorities engage in trade and commerce.
Gummow J in Minister for Immigration and Ethnic Affairs v Kurtovic71 noted the distinction drawn in the United States between proprietary and governmental capacities of public bodies. Where a public body acts in its proprietary capacity then an equitable estoppel may arise. His Honour drew an important distinction between the planning or policy level of decision-making by public authorities, in which statutory discretions are exercised, and operational decisions implementing such policy. He said:
Where the public authority makes representations in the course of implementation of a decision arrived at by the exercise of its discretion, then usually there will not be an objection to the application of a private law doctrine of promissory estoppel. It must, however, be recognised that it may be difficult, in a given case, to draw a line between that which involves discretion and that which is merely 'operational'.72
It is a distinction which makes sense but which may be difficult of application.
In the same year as Kurtovicwas decided, some important observations concerning the availability of estoppel against the Executive were made by Mason CJ in Attorney-General (NSW) v Quin, where his Honour said:
The Executive cannot by representation or promise disable itself from, or hinder itself in, performing a statutory duty or exercising a statutory discretion to be performed or exercised in the public interest, by binding itself not to perform the duty or exercise the discretion in a particular way in advance of the actual performance of the duty or exercise of the power.73
Nevertheless, Mason CJ did not deny the availability of estoppel against the Executive arising from conduct amounting to a representation if holding the Executive to its representation would not significantly hinder the exercise of the discretion in the public interest. He said:
… as the public interest necessarily comprehends an element of justice to the individual, one cannot exclude the possibility that the courts might in some situations grant relief on the basis that a refusal to hold the Executive to a representation by means of estoppel will occasion greater harm to the public interest by causing grave injustice to the individual who acted on the representation than any detriment to that interest that will arise from holding the Executive to its representation and thus narrowing the exercise of the discretion.74
The application of estoppels in public law is not foreclosed by the current state of authority in Australia.
The doctrine of legitimate expectations which attract particular requirements of procedural fairness in some cases bears some resemblance to estoppel but is not itself an equitable doctrine. Nor is it a species of estoppel. In particular, in Australia, it does not afford substantive protection to the rights the subject of the claimed expectation. As with the application of estoppel to the exercise of statutory discretions it would entail curial interference with administrative decisions on their merits by precluding the decision-maker from ultimately making the decision which he or she considered most appropriate in the circumstances. In Quin, Brennan J said of the concept of substantive protection:
That theory would effectively transfer to the judicature power which is vested in the repository, for the judicature would either compel an exercise of the power to fulfil the expectation or would strike down any exercise of the power which did not.75