Mutual wills are two wills made under a binding agreement that neither party will revoke or change their will without the other’s consent. Once one party dies without breaking the agreement, NSW courts can prevent the survivor from departing from it, even though technically nothing stops them from signing a completely different will.
Empower Probate Lawyers advises couples and families across NSW on structuring and disputing mutual wills agreements. This guide explains how the doctrine works, why identical wills alone are insufficient, and what happens when an agreement is breached.
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The Doctrine in Three Elements
Mutual wills are not simply a drafting style. They rest on a specific legal doctrine, sometimes called a contract not to revoke a will, which has three requirements that must all be satisfied.
The leading Australian authority is Birmingham v Renfrew [1937] HCA 52; (1937) 57 CLR 666, decided by the High Court. According to the High Court of Australia’s own case record, the case established the modern framework still applied today.
The doctrine requires:
- An agreement or contract between the two will-makers as to how their property will ultimately be distributed
- Survival of one party without revoking their will, meaning they kept their side of the bargain up to their death
- A constructive trust imposed on the survivor, preventing them from defeating the agreement once the first party has died
Why It Is Not Actually a Contract Claim
This distinction confuses many people, since the whole arrangement starts with an agreement.
Justice Dixon in Birmingham v Renfrew described the obligation as a “floating obligation, suspended during the lifetime of the survivor,” which then “crystallises into a trust” the moment the first party dies.
Beneficiaries cannot sue on the contract itself. What they can do is ask a court to enforce the constructive trust equity imposes to stop the survivor benefiting from breaking their word.
This matters practically. The survivor is generally still free to spend, use, or deal with the property as the absolute owner during their own lifetime. What they cannot do is leave what remains at their death to anyone other than the people the agreement promised it to.
Why Identical Wills Alone Are Not Enough
This is the single most common misunderstanding, and it catches out couples who assume matching wills automatically create a binding mutual wills arrangement.
NSW courts have repeatedly confirmed that simply making corresponding or identical wills does not, by itself, establish a binding agreement not to revoke them.
In Baird v Smee [2000] NSWCA 253, the NSW Court of Appeal applied the same principle Latham CJ set out in Birmingham v Renfrew: the mere fact that two wills correspond with each other, in the sense that each naturally explains the other, is not sufficient proof of a binding agreement.
Courts have described the burden of proving a genuine mutual wills agreement as heavy. Couples who actually want this level of protection generally need more than similarly worded wills to achieve it.
More: What Is the Statutory Legacy in NSW and How Much Does a Spouse Get?
What Actually Proves an Agreement Exists
Given how heavy the burden of proof is, couples wanting a genuine mutual wills arrangement should focus on clear, contemporaneous evidence rather than assuming the wills speak for themselves. Courts look for evidence created at the time the wills were made, not reconstructed years later once a dispute has already arisen.
- An express written agreement or deed, executed alongside the wills, is the strongest form of evidence and the approach most estate planning specialists recommend
- Clear wording within the wills themselves, expressly stating they are made pursuant to an agreement not to revoke
- Evidence of the circumstances of execution, such as both parties signing at the same time, in each other’s presence, with full knowledge of the other’s will
- Witness evidence, where available, confirming the parties explained the arrangement to family members or advisers at the time
An implied agreement can sometimes still be found without a written document, but this depends heavily on the strength of the surrounding evidence and is far harder to establish than an express agreement.
Families relying on an informal understanding, rather than a properly documented arrangement, take on a real risk that a court will later find no binding agreement ever existed.
Mutual Wills and Notional Estate in NSW
NSW has a distinctive statutory feature that most other jurisdictions do not, and it directly affects how mutual wills interact with family provision claims.
Under section 76(1)(f) of the Succession Act 2006 (NSW), entering into a contract disposing of property under a will is expressly listed as an example of a relevant property transaction. This means property covered by a mutual wills agreement can potentially be brought within the reach of a notional estate order under section 80 of the same Act.
In the earlier High Court decision Barns v Barns [2003] HCA 9, the Court held that property subject to a mutual wills contract was not automatically shielded from a family provision claim, even in the absence of notional estate-style legislation. NSW’s own statutory notional estate regime goes further, expressly contemplating this exact scenario.
More: What Is a Deed of Family Arrangement in NSW and How Does It Resolve Estate Disputes?
When Couples Actually Use Mutual Wills
Mutual wills are not the right structure for every couple, and understanding when they genuinely add value helps avoid using them where a simpler will would do.
Common situations where couples consider a mutual wills agreement include:
- Blended families, where both partners want to guarantee that assets ultimately reach their own children after the survivor’s death, rather than risk the survivor later favouring their own family exclusively
- Second marriages with existing wealth, where each partner wants certainty that their own assets pass to their intended beneficiaries in the end, even after benefiting the survivor first
- Business or farming families, where both parties have agreed how jointly built assets should ultimately be divided among the next generation
- Couples with no biological children in common, wanting to lock in an agreed distribution between two separate sets of beneficiaries
In each case, the appeal is certainty. Without a binding agreement, the surviving partner remains free to change their will however they like after the first death, which can defeat exactly the outcome both parties originally intended.
When a Mutual Wills Agreement Is Broken
Practical disputes usually surface only after the first party has died, once the survivor changes their will, remarries, or begins dealing with assets in a way that departs from what was agreed.
The Australian Law Reform Commission has described mutual wills as an ancient doctrine still carrying real modern consequences.
This is precisely because of this pattern: the promise is easy to make while both parties are alive, but the real test comes only when one of them is no longer there to enforce it personally.
The beneficiaries who were promised specific property under the original agreement can bring a claim seeking to have the court recognise a constructive trust over the remainder of the estate, even if the survivor’s later will says otherwise.
How Empower Probate Lawyers Can Help
Mutual wills sit at an unusual intersection of contract, equity, and succession law, which makes both the drafting and the later enforcement genuinely technical.
We advise couples on succession planning that properly documents an intended mutual wills arrangement, rather than relying solely on similarly worded wills. We also advise beneficiaries and executors on contesting a will where a mutual wills agreement appears to have been broken, and on family provision and will disputes more broadly across NSW.
Call 1300 481 161 for an initial consultation. Specialist advice. Free initial consultation. Deferred fee arrangements are available in eligible cases.
Frequently Asked Questions
Are mutual wills and mirror wills the same thing?
No. Mirror wills simply use similar or identical terms for convenience and can be changed freely by either party at any time. Mutual wills involve a binding agreement not to revoke, which courts will enforce by imposing a constructive trust upon one party’s death.
Can the surviving party spend the money left to them?
Generally, yes. The survivor is usually free to use and enjoy the property as the absolute owner during their own lifetime. The obligation is to what remains at their own death, not to preserve every asset untouched.
What happens if there is no written agreement?
Courts can still find an implied mutual wills agreement in some circumstances, but proving it without an express written agreement is significantly harder, since matching wills alone are not sufficient evidence.
Can a mutual wills agreement be changed later?
Yes, if both parties agree. The doctrine restrains unilateral departure from the agreement, not a genuine, mutually consented variation made while both parties are still alive.
Can a family provision claim still succeed against a mutual wills estate in NSW?
Yes. NSW’s notional estate provisions specifically allow the Court to treat property covered by a contract disposing of property under a will as available for a family provision claim in appropriate circumstances.
Do mutual wills need to be identical to each other?
No. While many mutual wills mirror each other’s terms, particularly when spouses leave everything to one another, the doctrine focuses on the existence of a binding agreement, not on the wills’ identical wording.
Is a mutual wills agreement the same as a deed of family arrangement?
No. A mutual wills agreement is made between two will-makers while both are alive, to govern how each will can later be changed. A deed of family arrangement is made by beneficiaries after death to vary the distribution of an existing estate, often used to settle a dispute rather than prevent one.
Can one party revoke a mutual wills agreement while both are still alive?
Generally, yes, provided the other party is given proper notice. The binding obligation not to depart from the agreement typically crystallises only upon the death of a party who has not revoked their side of the arrangement.
