If you have ever offered a cash reward for returned equipment, promised a referral credit, or built a sales bonus into your team's incentive plan, you have probably created a unilateral contract — whether you knew it or not. These arrangements work differently from the everyday contracts your business signs, and that difference has real consequences for when you are bound, when you can walk away, and what happens when a dispute arises.
This article explains what a unilateral contract is, how it forms under Australian law, and where businesses routinely come unstuck. It covers:
- the essential legal elements
- how acceptance and revocation work in practice
- where these contracts appear in everyday business operations
- the most common mistakes and misconceptions
- when to get legal help before problems arise
What a unilateral contract is
A unilateral contract is one where a single party makes a promise that can only be accepted by the other side actually doing something — not by agreeing, signing, or promising to do it. The promisor is only obliged to perform once the specified act is completed.
The clearest illustration is a reward offer: "We will pay $500 to anyone who returns our stolen laptop." Nobody you say that to is obliged to search for the laptop. But the moment someone hands it back, the contract is formed and the obligation to pay crystallises. Acceptance occurs through performance, not through words.
This stands in contrast to a bilateral contract, where both parties exchange promises and are bound from the moment they agree. A supplier contract, a services agreement, or a lease — these are bilateral: each side commits to something, and the binding moment is the exchange of those commitments. In a unilateral arrangement, only one party is ever truly bound, but their obligation is triggered automatically by someone else's act.
The elements that make a unilateral contract enforceable
A unilateral contract must satisfy the same basic requirements as any other contract in Australia. If any element is missing, the arrangement may not be binding.
Offer. The offer must be clear and certain. "We might pay something for useful information" is not an offer — it is an invitation to negotiate. A valid unilateral offer specifies what the promisor will do, what act the other person must perform, and ideally any limits (timeframe, quantity, eligibility). Vagueness is fatal. Courts will not enforce what they cannot understand, and ambiguity invites disputes about whether the conditions were met.
Acceptance by performance. The other party accepts by completing the act — not before. No communication of acceptance to the offeror is required; the performance itself is sufficient.
Knowledge of the offer. This is an element that catches many businesses off guard. The person who performs the act must have known about the offer at the time they performed it. If someone happens to do exactly what your offer requested, but had no idea the offer existed, there is no acceptance in the legal sense and no contract is formed. Courts in Australia have consistently treated knowledge as a precondition to acceptance: a person cannot accept an offer they did not know about.
Consideration. In a unilateral contract, the act itself constitutes consideration. The offeror promises something of value; the offeree provides that value by performing. The act does not need to be onerous — it simply needs to be the thing that was requested.
Intention to create legal relations. In commercial and business settings, courts presume that parties intend to be legally bound. That presumption works in your favour when you want the offer enforced, but it also means that loosely worded promotional promises can create genuine legal obligations even if you thought of them as informal.
Certainty. The terms must be sufficiently clear that a court can determine what each party is entitled to. A promise that leaves the essential terms — what must be done, what will be paid, by when, and to whom — undefined or at large will not be enforceable.
How acceptance and revocation actually work
Accepting by performance
The moment someone completes the required act — knowing about the offer and meeting any stated conditions — acceptance is complete and the contract is formed. At that point, the offeror's promise becomes binding. There is no further step required.
This immediacy is one reason that precise drafting matters. If your offer is ambiguous about what act qualifies, you may face a genuine dispute about whether acceptance occurred at all, or whether the act performed was the act you intended to reward.
Revoking a unilateral offer
Technically, an offeror may revoke a unilateral offer at any time before acceptance — that is, before the required act is completed. But "technically" conceals some important practical complexity.
Australian courts have not adopted a blanket rule that commencing performance automatically locks an offer in place. However, they have recognised that circumstances can arise where an implied obligation not to revoke emerges once a person has embarked on performance in reasonable reliance on the offer. Depending on the facts, doctrines such as equitable estoppel may come into play if someone has materially changed their position based on your offer and you then withdraw it without notice.
The practical upshot: if you run a promotion, referral scheme, or incentive program, you should state clearly in writing how and when the offer can be withdrawn, and what notice — if any — you will give before it closes. Silence on revocation is an invitation for arguments you do not want to have.
Where unilateral contracts arise in business operations
Public reward and recovery offers
"We will pay $1,000 to anyone who provides information leading to the return of our stolen equipment." This is a textbook unilateral offer. The obligation to pay crystallises when someone who knew about the offer provides information that actually leads to the recovery — assuming your terms specify "leading to the return" clearly enough to be tested.
Referral and loyalty programs
"Refer a friend who becomes a paying customer and receive a $100 account credit." These programs are usually unilateral: nobody is obliged to refer anyone, but once the referral results in a paying customer, the credit is owed. The critical issues are definition (what counts as a referral? what counts as a "paying customer"?) and timing (when exactly is the credit applied?). Ambiguity in either creates disputes.
Referral programs typically involve collecting personal information — names, email addresses, phone numbers. Businesses that handle personal information are subject to the Privacy Act 1988 (Cth) and the Australian Privacy Principles, which require that you tell people what you collect, why you collect it, and how it will be used. A privacy policy is not optional if you are collecting information as part of a referral scheme.
Performance and sales bonuses
"You will receive a bonus payment if your team exceeds $150,000 in quarterly sales." If this is framed purely as a public or team-wide contingent promise, it can operate as a unilateral contract. However, where the bonus is part of an employment arrangement, it is better handled through a written employment contract or a documented bonus policy that specifies the measurement period, the qualifying figure, and the payment date. Unilateral bonus promises without documentation are a reliable source of employment disputes.
Service guarantees and promises
"If we fail to respond within four business hours, your next service call is free." The customer's entitlement only arises if the condition occurs. That is the structure of a unilateral contract. These guarantees need to comply with the consumer guarantees regime under the Australian Consumer Law (Sch 2 of the Competition and Consumer Act 2010 (Cth)), and the conditions for triggering the guarantee must be drafted with precision.
Common mistakes and where disputes arise
Treating the offer as informal
Because unilateral offers can be made in a public post, a printed flyer, or a single social media caption, businesses sometimes treat them as aspirational marketing rather than legal commitments. Courts do not share that view. If your offer is certain enough to be understood, it is likely certain enough to be enforced. The casualness of the medium does not diminish the legal effect.
Burying conditions in fine print
Section 18 of the Australian Consumer Law prohibits engaging in conduct that is misleading or deceptive, or is likely to mislead or deceive, in trade or commerce. Promoting a reward or offer prominently while concealing the eligibility conditions in fine print can breach this provision — even if you never intended to mislead anyone. Intention is not a defence. What matters is the impression created in the minds of those to whom the offer is directed. Conditions must be prominent, clear, and accessible.
Attempting to revoke mid-performance
If you pull a promotion or reward scheme while people are actively partway through completing the required steps, you risk a dispute about whether you could revoke at that point, and whether the individuals who had partly performed have any claim in reliance. State clearly when and how the offer can be terminated, and give reasonable notice before withdrawal where the circumstances call for it.
Confusing unilateral mistake with the contract being void
A unilateral mistake — where one party is mistaken about a fundamental term and the other is not — does not automatically void a contract in Australia. The High Court in Taylor v Johnson [1983] HCA 5; (1983) 151 CLR 422 confirmed that a contract formed under a unilateral mistake will generally stand unless equity intervenes. Where the other party knew of, or ought reasonably to have known of, the mistake and proceeded anyway — particularly in a way that took advantage of it — a court may grant relief on the basis of unconscionability. This is a fact-specific assessment. Simply discovering after the fact that your offer was more generous than you intended is unlikely to be enough.
How an Artificer Legal solicitor can help
Unilateral offers are straightforward in concept but legally sensitive in execution. A solicitor can assist you with:
Drafting enforceable offers. Translating a business idea — a referral program, a performance bonus, a service guarantee, a public reward — into language that is clear enough to be enforced but precisely limited enough to protect you from claims you did not intend to take on. This means specifying the qualifying act, any exclusions, any caps or timeframes, and the claims process.
Reviewing existing promotions and incentive programs. If you are already running a reward scheme, referral program, or bonus arrangement, a solicitor can assess whether the terms are clear and compliant with the Australian Consumer Law and whether your privacy practices meet the requirements of the Privacy Act 1988 (Cth).
Advising on revocation and variation. If you need to change or end an ongoing offer, the timing and method of withdrawal matter. A solicitor can advise on whether a proposed withdrawal is defensible and what steps to take to minimise reliance-based claims.
Resolving disputes. Where a claimant asserts that they accepted your offer by performance and you disagree, the dispute turns on the precise wording of the offer, the evidence of the act performed, and whether the knowledge and certainty requirements were met. Those are questions for a solicitor with contract expertise, not something to resolve through a payment made to avoid a scene.
Artificer Legal works with Australian SMBs on business contracts and commercial disputes. If you are designing an incentive program, reviewing a promotion, or dealing with a claim arising from an offer you made, we can help you understand where you stand and how to resolve it.
Getting the fundamentals right
A unilateral contract is a promise that becomes binding the moment someone performs the required act. The person performing must have known about the offer, the offer must be certain enough to be understood, and the act must actually be what the offer called for.
Key points to carry away:
- There is no acceptance without knowledge: if the person did not know about your offer when they acted, you are not bound.
- Starting performance does not automatically lock your offer in place, but reliance by the other party can limit your ability to withdraw cleanly.
- Public offers and promotions must not mislead, and conditions must be prominent — s 18 of the Australian Consumer Law applies regardless of intent.
- Referral and loyalty programs that collect personal data are subject to the Privacy Act 1988 (Cth) and the Australian Privacy Principles.
- In a dispute about unilateral mistake, the question is not just whether one party was wrong — it is whether the other party knew about it and took advantage of the situation.
The simplest protection is precise, accessible drafting from the outset, housed in the right document and reviewed before you go public.