Estoppel 

Estoppel prevents a party from departing from an assumption another has adopted and acted on, where departure would be unconscionable. Its importance in Australia is out of proportion to its size, because Australian law took it further than England did.

The varieties

Common law estoppel — including estoppel by representation and estoppel by convention — operates on an assumption of existing fact. It is evidentiary in character: it prevents the denial of the assumed state of affairs, and the rights of the parties are then determined on that footing. It does not itself create a cause of action.

Equitable estoppel operates on an assumption about the future, typically induced by a promise. It divides conventionally into:

• promissory estoppel, concerning an assumption that a legal relationship will be altered or a right not enforced; and
• proprietary estoppel, concerning an assumption that the claimant has or will receive an interest in the promisor's property.

Whether these are aspects of one unified doctrine of equitable estoppel is unsettled in Australia. Judicial opinion has favoured unification at various points without settling it, and the safer course is to identify the category and note the debate rather than to assert a single doctrine.

A related question arose in Orr v Ford (1989) 167 CLR 316², decided on laches. Deane J (with whom Mason CJ agreed on laches) said that it may be that the developing scope of estoppel by conduct is leading to a unification of doctrine in areas such as laches, but the question had not been argued and could be left for another case. The joint reasons of Wilson, Toohey and Gaudron JJ said, without deciding it on the facts, that where a beneficiary's inactivity had induced a belief in the holder of the property on which the holder acted, the matter would be one of estoppel and not laches, and an estoppel of that nature could be raised against the beneficiary's claim.

The Australian expansion

The orthodox English position confined promissory estoppel to the defensive modification of an existing legal relationship — a shield, not a sword. Australian law departed from this in Waltons Stores (Interstate) Ltd v Maher (1988) 164 CLR 387,¹ where the High Court accepted that equitable estoppel may found relief where there was no pre-existing legal relationship at all, so a promise unsupported by consideration may generate an enforceable claim if the other elements are satisfied.

This is why estoppel must be considered whenever a promise fails for want of consideration. It is not a general enforcement of promises — the elements below do real work, and the relief is not automatically the promise — but it is a genuine alternative route, and the contrast with the consideration doctrine should be drawn expressly.

Elements

1. The claimant assumed or expected that a particular legal relationship exists or will exist, or that an interest will be granted.
2. The defendant induced the claimant to adopt that assumption, or is otherwise responsible for it — by promise, encouragement, or knowing silence where there was a duty to speak.
3. The claimant acted or refrained from acting in reliance on the assumption.
4. The defendant knew or intended that the claimant would do so.
5. The claimant will suffer detriment if the assumption is departed from.
6. The defendant failed to act to avoid that detriment, whether by fulfilling the assumption or otherwise.

Detriment is assessed in the round at the time of departure, not transaction by transaction, and it need not be financial: foregoing other opportunities, changing life plans, or working for years on a property in expectation of inheriting it will suffice. Unconscionability is not a seventh element floating free of the others; it is the conclusion the elements support.

For an estoppel arising by encouragement from a promise, where the claimant asserts new property rights, the High Court has refined those requirements. In Kramer v Stone [2024] HCA 48³ the joint reasons of Gageler CJ, Gordon, Edelman and Beech-Jones JJ (the appeal was dismissed, four Justices to one) set out four elements: a clear and unequivocal promise; a reasonable expectation or intention (or the promisor's actual expectation or intention) that the promisee would rely on it; reliance in the general manner expected; and detriment if the promise is not fulfilled ([36]-[40]). They held that no further encouragement after the promise, and no actual knowledge of the promisee's reliance, is required ([34]-[35]). Gleeson J dissented, holding that conduct after the promise encouraging reliance was required ([68]). The first of those elements has been queried: in Kronenberg v Macaulay [2025] NSWCA 195⁴ Leeming JA (Mitchelmore and Free JJA agreeing) saw "some difficulties" in requiring a clear and unequivocal promise where the case is one of proprietary estoppel by encouragement from a promise, but did not decide the point and did not depart from Kramer. State the joint reasons' requirement and say that it has been queried, without decision, in the New South Wales Court of Appeal, which has since said that Kramer must be followed there (Penya v Penya [2026] NSWCA 73⁵).

Measuring the relief

Two measures compete:

• the expectation measure, making good the assumption — transferring the property or enforcing the promise; and
• the reliance measure, the minimum equity to do justice — compensating the detriment suffered.

Australian authority has moved towards fulfilling the expectation in proprietary estoppel cases, unless doing so would be disproportionate to the detriment, in which case the court moulds lesser relief. In promissory estoppel the reliance measure is more prominent. Relief remains discretionary and is shaped to the case: a monetary equivalent, an interest in land, a lien, or an order postponed in time are all available.

Applying this in a problem question

1. Identify the assumption precisely, and whether it concerns existing fact or the future.
2. Name the category, and note the unresolved unification question if it matters.
3. Work through the elements in order, spending the most effort on inducement and detrimental reliance.
4. Assess detriment in the round at the moment of departure from the assumption.
5. Argue the measure of relief explicitly rather than assuming the promise will be enforced.

Where the authority sits

Case law. The Australian departure from the English confinement of promissory estoppel, and the measure of relief in proprietary estoppel, are both High Court developments and both should be attributed to Australian authority rather than to English texts.