'The following can I think be distilled from the reasons in Waltons notwithstanding the somewhat different language used by the different judges: (1) Common law and equitable estoppel are separate categories, although they have many ideas in common; (2) Common law estoppel operates upon a representation of existing fact, and when certain conditions are fulfilled, establishes a state of affairs by reference to which the legal relation between the parties is to be decided. This estoppel does not itself create a right against the party estopped. The right flows from the court's decision on the state of affairs established by the estoppel; (3) Equitable estoppel operates upon representations or promises as to future conduct, including promises about legal relations. When certain conditions are fulfilled, this kind of estoppel is itself an equity, a source of legal obligation; (4) Cases described as estoppel by encouragement, estoppel by acquiescence, proprietary estoppel and promissory estoppel are all species of equitable estoppel; (5) For equitable estoppel to operate in circumstances such as those of the present case there must be the creation or encouragement by the defendant in the plaintiff of an assumption that a contract will come into existence or a promise be performed, and reliance on that by the plaintiff, in circumstances where departure from the assumption by the defendant would be unconscionable; (6) Equitable estoppel may lead to the plaintiff acquiring an estate or interest in land, that is, in the common metaphor, it may be a sword; (7) The remedy granted to satisfy the equity (which either is the estoppel or created by it) will be what is necessary to prevent detriment resulting from the unconscionable conduct."
10 The proposition 5 in that passage was expanded by his Honour Priestley JA in the quote from Austotel which has been earlier set out. It is that formulation which seems to me to be most pertinent to the circumstances of this case.
11 Doyle CJ of the Supreme Court of South Australia in Mortgage Acceptance Nominees Ltd v Australian Thoroughbred Finance Pty Ltd (1996) 69 SASR 302 at 304 referred to the High Court decisions of Waltons Stores (Interstate) Ltd v Maher (supra) and Commonwealth v Verwayen (supra). His Honour then continued:
"The judgments in those cases on a number of occasions make it clear that it is unwise to attempt to state in a comprehensive and definite fashion the manner in which a promissory estoppel may be established. There are two reasons for this. The first is that the relevant principle is founded upon the prevention of unconscionable conduct, and experience has taught us that equitable principles founded upon such notions defy precise definition. The second reason is that the law in this area is still developing, and an attempt at precision may give a false sense of certainty."
12 Notwithstanding those factors, his Honour referred to two passages in the joint judgment of Mason CJ and Wilson J in Waltons v Maher (supra). In the first of these passages their Honours observed at 399:
"Promissory estoppel certainly extends to representations (or promises) as to future conduct ... So far the doctrine has been mainly confined to precluding departure from a representation by a person in a pre-existing contractual relationship that he will not enforce his contractual rights, whether they be pre-existing or rights to be acquired as a result of the representation.
However, at 406 their Honours said:
"The foregoing review of the doctrine of promissory estoppel indicates that the doctrine extends to the enforcement of voluntary promises on the footing that a departure from the basic assumptions underlying the transaction between the parties must be unconscionable. As failure to fulfil a promise does not of itself amount to unconscionable conduct, mere reliance on an executory promise to do something, resulting in the promisee changing his position or suffering detriment, does not bring promissory estoppel into play. Something more would be required. Humphreys Estate [[1987] AC 114]] suggests that this may be found, if at all, in the creation or encouragement by the party estopped in the other party of an assumption that a contract will come into existence or a promise will be performed and that the other party relied on that assumption to his detriment to the knowledge of the first party."
13 Further, in Anderson v Commonwealth Bank of Australia (29 September 1995, unreported) Lindgren J was concerned with circumstances where Mr and Mrs Anderson, having taken part in an arbitration pursuant to an arbitration agreement, sought to have that agreement set aside and have their claims litigated in the Federal Court. His Honour held that they had elected in favour of the arbitration agreement, and against its being rescinded or set aside, but his Honour also found that, in the alternative to having made an effective election:
"…Mr and Mrs Anderson are estopped from asserting the case which they propound for the setting aside of the Arbitration Agreement and the Arbitration Award. By their conduct from 14 June 1994 down to the filing of their Amended Application on 13 June 1995 at the earliest, they led the Bank to assume that they were treating themselves as bound by the arbitration process and its result. To the Andersons' knowledge, the Bank acted on that assumption to its detriment by incurring the substantial costs associated with the arbitration (fees of solicitors and senior and junior counsel of a six-day hearing and of written submissions down to 9 March 1995, the arbitrator's fees and cost of transcription services which the Arbitration Agreement required the Bank to bear, and the time of the Bank's staff) and it would be unconscionable for Mr and Mrs Anderson now to renege. For the proposition that an estoppel arises in such circumstances many authorities could be cited, but it suffices to refer to Waltons Stores (Interstate) Ltd v Maher (1988) 164 CLR 387."
14 On this appeal, counsel for Mr Horsman contends that there was no evidence that the respondent assumed that the proceedings before the Commissioner would preclude the appellant from taking further proceedings, or that Mr Horsman knew of that assumption. It is further submitted that there was no evidence that Mr Horsman understood the union's undertaking to bind him. It was also submitted on behalf of Mr Horsman that there could be no legal relationship between the appellant and the respondent, since Mr Horsman was not a party to the proceedings before Commissioner Laing.
15 It was also sought to be contended on behalf of Mr Horsman that there was insufficient material from which a clear inference could be drawn that the respondent relied on the assumption. In particular it was pointed out by counsel for Mr Horsman that the respondent gave no such evidence before the learned primary judge, and in particular, no evidence as to its knowledge or its reliance. It was further submitted that there was no evidence from which to conclude that Mr Horsman knew or intended that the commissioner for Main Roads acted in reliance on the assumption, nor that the Commissioner for Main Roads understood the appellant to be acquiescing in the undertaking that had been offered before the Commission. It was submitted that there was no evidence to show that the respondent was aware that he was not compelled to participate in the proceedings but did so because of the appellant's acquiescence. It is also finally submitted that there was a lack of proportion between the detriment suffered by Mr Horsman and holding in favour of the respondent on the question of estoppel.
16 In those circumstances it is necessary to have regard to the evidence in some detail.
17 First, in a letter dated 9 June 1992, Mr Bruce Wilson, the branch secretary of the Australian Workers' Union, West Australian branch, wrote to Mr Horsman. The letter was headed "Re Your Unfair Dismissal Claim" and referred to an unsigned letter dated 28 May 1992. That letter was not in evidence before either the learned primary judge or this court. The body of Mr Wilson's letter is in the following terms:
"I refer to your unsigned letter dated the 28th May 1992.