In short

Mutual wills are a legal mechanism, rooted in the High Court case Birmingham v Renfrew, where both partners agree the survivor's will cannot be changed after the first death to alter agreed dispositions to blended-family children. The survivor keeps full lifetime use of the assets, but a constructive trust enforces the original agreement after their death. The commitment requires clear documentation and genuine mutual willingness to be bound.

For Australian retirees in second marriages or blended-family situations — a couple where one or both partners have adult children from prior relationships — one of the most common estate-planning concerns is structural. The typical pattern: each partner makes a will leaving everything to the other on the first death, with a verbal understanding that the survivor will eventually leave the combined assets equally (or in some agreed proportion) among all the children. He has two children from his first marriage; she has three from hers; they expect that the eventual estate, after both have died, will be shared among all five. The trouble is that without a binding legal mechanism, the surviving spouse — having inherited everything from the first to die — is generally free to change their own will at any time (MoneySmart — wills, https://moneysmart.gov.au/wills-and-powers-of-attorney/wills, accessed 11 May 2026). New family pressures, new relationships, evolving family dynamics, or simply changes of mind can produce a survivor's later will that favours their own biological children at the expense of the deceased's biological children. The deceased's expectation that his wealth would flow eventually to his children is defeated, and the consequence is often lasting family bitterness that the deceased never wanted.

Mutual wills are a specific legal mechanism developed by the courts to address this structural problem. The arrangement is more than simply "wills made at the same time" — joint wills (a single document) and simultaneously-executed individual wills don't automatically create mutual wills obligations. The defining feature is the agreement not to revoke: two parties (typically spouses or de facto partners) agree to make reciprocal wills and agree that the survivor will not revoke or change their will in a manner inconsistent with the agreement after the first death. The Australian doctrine is rooted in the High Court's decision in Birmingham v Renfrew (1937) 57 CLR 666 (https://classic.austlii.edu.au/au/cases/cth/HCA/1937/52.html, accessed 11 May 2026), where the Court held that property received under such an agreement is held by the survivor subject to a "floating obligation" that crystallises on the survivor's death — any later inconsistent will is challengeable, and constructive trust is available to ensure the originally agreed dispositions are honoured. The doctrine has been applied and refined in subsequent Australian decisions and remains the operative framework today.

The formation requirements for an enforceable mutual wills arrangement center on clear evidence of the agreement not to revoke. Courts have found mutual wills based on express written agreements between the testators (the strongest evidence), recitals within the wills themselves stating the mutual obligation (clear textual evidence), and strong contextual evidence such as correspondence and witness testimony (less strong but sometimes sufficient). The mutual obligation must bind both parties — a unilateral commitment by one party isn't a mutual will arrangement; both must agree to be bound and both must make wills consistent with the agreement. For practical estate planning, the safest approach is an explicit written agreement (sometimes called a Mutual Wills Agreement or Mutual Wills Deed) signed by both parties, expressly setting out the terms and the binding obligation, alongside the actual wills which should also recite the mutual wills commitment (Law Council of Australia — wills and estates practice resources, https://www.lawcouncil.au/policy-agenda/legal-practice/wills-and-estates, accessed 11 May 2026).

The enforcement mechanism operates through equity rather than the will itself. Under Birmingham v Renfrew, when the first party dies and the survivor inherits assets under the mutual will arrangement, those assets are subject to a floating obligation in favour of the agreed ultimate beneficiaries. The survivor has full beneficial use of the assets during their lifetime — they can spend, invest, dispose, enjoy as they choose — but the post-death testamentary disposition is constrained. The floating obligation crystallises into a constructive trust upon the survivor's death, fixing on whatever assets remain. If the survivor has made a will inconsistent with the mutual wills agreement, the originally agreed beneficiaries can challenge the survivor's estate after their death, seeking to enforce the agreement through the constructive trust. The remedy operates against the survivor's estate, brought by the originally agreed beneficiaries (typically the deceased first party's children whose share was reduced under the survivor's later will).

What the survivor can and cannot do under a mutual wills arrangement is sometimes misunderstood. The survivor can use, spend, invest, and consume the inherited assets during their lifetime — mutual wills don't lock the assets away as if held in a fixed trust during life. They can make changes to their will that don't contradict the mutual wills commitment, such as updating executors, refreshing personal effects bequests, or making minor changes that don't affect the principal dispositions. What they cannot do is change the will in a way that affects the originally agreed dispositions to the named beneficiaries. They generally cannot dissipate assets specifically to defeat the agreement (such as gifting away most of the estate during life to avoid the mutual wills constraint) — Birmingham v Renfrew specifically contemplates that the survivor cannot deliberately defeat the arrangement, though ordinary lifetime use within reasonable bounds is permissible. The constraint is on testamentary disposition rather than lifetime control.

For blended-family retirees, comparing mutual wills with alternatives is part of the planning conversation. Testamentary trusts are trust structures created by the will and operating after death; they don't bind the survivor's testamentary choices but provide asset-management protection and tax efficiency for beneficiaries. Life interests give the survivor a right to use and benefit from property during their lifetime, with remainder to specified beneficiaries; the disposition is bound but the structure creates ongoing administrative complexity (separate ownership, reversion accounting, sometimes tax inefficiencies). Family Provision claims under each state's succession legislation are the statutory framework allowing eligible persons (spouses, children, certain dependants) to challenge wills perceived as inadequate; they don't pre-bind the testator but provide remedy after the fact (Legal Aid NSW — making a will, https://www.legalaid.nsw.gov.au/my-problem-is-about/wills-estates-and-funerals/making-a-will, accessed 11 May 2026). Mutual wills sit among these as a specific mechanism that pre-binds testamentary choices through equitable obligation while leaving the survivor with full lifetime ownership and use. The choice depends on the specific objectives, family dynamics, asset structure, and willingness of the parties to be bound.

The risks and limitations of mutual wills should be understood by parties considering the arrangement. Inflexibility is the principal trade-off — the binding commitment doesn't adapt to changed circumstances over what may be decades between first and second deaths. New family relationships (the survivor remarrying, new grandchildren, family disputes) cannot easily be accommodated within the locked-in disposition pattern. Tax planning constraints can arise — the survivor may face restrictions on estate planning that would benefit all beneficiaries because the mutual wills constraint limits their flexibility. Family Provision claims by eligible persons (a new spouse of the survivor, for example) may proceed regardless of the mutual wills agreement — the equitable obligation doesn't immunise the eventual estate against statutory claims under each state's succession Act, and a successful family provision claim takes priority over the mutual wills disposition. Evidence challenges can arise where the mutual wills agreement isn't well-documented, with courts sometimes finding insufficient evidence to support enforcement. Lifetime dissipation by the survivor can reduce the eventual estate that's ultimately available for distribution, though specific dissipation aimed at defeating the agreement is challengeable on equitable grounds.

The practical drafting and execution of mutual wills should involve a specialist estate planning solicitor. The recommended approach is a separate Mutual Wills Agreement document, signed by both parties, expressly setting out the binding commitment, the assets and dispositions covered, and the agreed beneficiaries — this provides the clearest evidence of the agreement that Birmingham v Renfrew requires. The wills themselves should recite the mutual wills agreement in their preamble or recitals, with cross-reference to the separate agreement. The wills should be witnessed properly under the relevant state's wills legislation and stored together with the agreement. Periodic review is appropriate as life events occur (births, deaths, family disputes, changes in asset positions) — though the mutual wills cannot be unilaterally changed by either party, both parties together can vary the agreement if both consent. After the first death, the survivor cannot vary the agreement; the dispositions are locked in.

The integration with super and broader estate planning matters substantially. Super death benefits don't pass under the will — they pass via Binding Death Benefit Nomination (BDBN) under SIS Regulation 6.17A (https://classic.austlii.edu.au/au/legis/cth/consol_reg/sir1994582/s6.17a.html, accessed 11 May 2026) or trustee discretion in the absence of a binding nomination. Mutual wills don't directly bind super death benefits. For blended-family retirees, the integrated estate plan needs to coordinate the will (with mutual wills if appropriate), the BDBN for super (the operational mechanics of which are covered at articles/2026-05-05-bdbn-lapse-3-year-rule), life insurance beneficiary nominations, and joint property ownership arrangements. The super piece is often the larger asset for many retirees, and the BDBN coordination matters as much as the will.

For practitioners advising blended-family retiree clients, the mutual wills conversation typically arises when the client expresses concerns about what happens to their children's inheritance after they die — particularly the concern that the surviving spouse might change the will to favour their own biological children. Mutual wills are one option in the menu of solutions, alongside testamentary trusts, life interests, separate property arrangements, and other mechanisms. The choice depends on the client's specific concerns, the willingness of both partners to be bound, the nature of the assets, and the family dynamics involved. For couples with genuine mutual commitment to the agreed dispositions and willingness to be bound, mutual wills provide clear and enforceable protection. For couples where one party wants flexibility for the future, alternative mechanisms may be more practical.

What do worked planning examples show?

These two cases show how mutual wills play out for typical blended-family scenarios. Illustrative only — not personal advice — using FY25-26 figures.

Case 1 — Robert (72) and Margaret (68), in their second marriage. Robert has 2 adult children from his first marriage; Margaret has 3. Combined assets approximately $2.2m: a home worth $1.3m owned jointly, super of $400k each, investments of $100k. They want their respective children to inherit fairly from their combined estate after both have died, with the survivor supported during their lifetime. On these facts, the rational pathway involves engaging an estate planning solicitor specialising in mutual wills to draft a Mutual Wills Agreement and reciprocal wills aligned with Birmingham v Renfrew principles. The agreement specifies that on the first death, the survivor inherits all assets; on the second death, the residual estate is divided in a specified way among all 5 children (perhaps equal shares, or shares reflecting their biological parent's prior contributions). Both Robert and Margaret sign the agreement, confirming the binding commitment. They coordinate the BDBN for their super to align with the mutual wills disposition (super flows to surviving spouse on first death; the second-to-die's super flows under a binding nomination consistent with the will). The trap to avoid is informal verbal agreement without documentation — courts won't enforce mutual wills based purely on testimony of a verbal understanding, particularly years after the agreement, and the evidentiary standard from Birmingham v Renfrew onwards requires clear proof of the agreement not to revoke.

Case 2 — David (76), widowed for 8 years, recently remarried to Susan (62). He has 3 children and 5 grandchildren from his first marriage. Susan has 1 adult child. David wants Susan supported during her lifetime if he dies first, but his estate ultimately to flow to his children and grandchildren. Susan wants flexibility to provide for her own daughter from her existing assets. On these facts, mutual wills may not be the right mechanism — Susan's reluctance to be bound and the significant difference in their respective family structures make the mutual commitment difficult. The alternative pathway involves a life interest for Susan in David's principal home (right to occupy during her lifetime) with remainder to David's children, and a separate disposition of David's other assets via testamentary trust for his children and grandchildren. Susan retains her own assets and her own testamentary freedom, but doesn't get David's assets to dispose of. The trap to avoid is using mutual wills where the parties don't have genuine mutual commitment — the documentation and effort don't produce real protection if a court later finds insufficient evidence of binding agreement, and the relationship friction during the parties' lifetimes (one party feeling locked in, the other feeling distrusted) can undermine the marriage itself.

For Australian blended-family retirees, mutual wills are a specific legal mechanism — anchored in Birmingham v Renfrew and the equity courts' constructive trust jurisdiction — that addresses the structural concern of the surviving spouse changing the will after the first death to favour their own biological children. The arrangement creates an equitable obligation enforceable by the courts, protecting the originally agreed dispositions to all beneficiaries. The mechanism requires genuine mutual commitment, careful documentation, and integration with broader estate planning including super BDBNs. For couples with strong shared commitment to the agreed outcomes, mutual wills provide clear and binding protection. For couples where one party wants future flexibility, alternative mechanisms (life interests, testamentary trusts, separate property arrangements) may be more practical. The advice work is to surface the mechanism, evaluate its fit with the client's specific circumstances, and engage specialist estate planning legal support for the drafting and execution.

Sources

Key takeaways

  • Mutual wills require an explicit agreement not to revoke — simply making wills at the same time, or even a joint will, doesn't automatically create a binding mutual wills obligation without clear evidence of that agreement.
  • Under Birmingham v Renfrew (1937) 57 CLR 666, assets the survivor inherits under a mutual wills arrangement are subject to a 'floating obligation' that crystallises into a constructive trust on the survivor's death, enforceable by the originally agreed beneficiaries.
  • The surviving partner retains full lifetime use, spending, and investment freedom over the inherited assets — the constraint applies only to their testamentary disposition after death, not to how they live with the assets while alive.
  • Family Provision claims by eligible persons, including a new spouse of the survivor, can still succeed against a mutual wills estate and take priority over the mutual wills disposition — the arrangement doesn't immunise the estate against statutory challenge.
  • The safest way to create an enforceable mutual wills arrangement is a separate written Mutual Wills Agreement signed by both parties, with the wills themselves reciting the commitment, prepared with a specialist estate planning solicitor.

Frequently asked questions

What are mutual wills and how do they differ from ordinary wills?

Mutual wills involve an explicit agreement between two partners that the survivor will not revoke or change their will after the first partner dies, in a way that's inconsistent with the agreed dispositions. Ordinary wills, even if made at the same time as a partner's, don't carry this binding legal effect — the survivor is free to change their will at any time unless a genuine mutual wills agreement exists.

Can a surviving spouse spend the inheritance under a mutual wills arrangement?

Yes. The survivor retains full use of the inherited assets during their lifetime and can spend, invest, or dispose of them as they choose. The mutual wills constraint applies only to how they leave the remaining assets in their own will after their death, not to their lifetime use of the assets.

Can someone challenge a mutual wills arrangement after the survivor dies?

Yes, in two ways. The originally agreed beneficiaries can enforce the arrangement through a constructive trust if the survivor's later will breaches the agreement. Separately, eligible persons — such as a new spouse of the survivor — can still bring a Family Provision claim under state succession law, and a successful claim can take priority over the mutual wills disposition.

How do I make a mutual wills agreement enforceable?

The strongest approach is a separate written Mutual Wills Agreement, signed by both parties, that clearly sets out the binding commitment, the assets covered, and the agreed beneficiaries, with the wills themselves reciting that agreement. Courts generally won't enforce mutual wills based only on a verbal understanding, so proper documentation with a specialist estate planning solicitor is essential.

A note on advice. This article is general information only and doesn't account for your personal circumstances. Everyone's situation is different — before acting, it's worth talking it through with a licensed adviser who knows your full picture.