If a loved one had dementia, Alzheimer’s, or another condition affecting their memory and judgment when they made or changed their will, you may have grounds to challenge it. This isn’t about whether the will feels fair — it’s about whether the will-maker legally had the mental capacity to make the decisions in it. Here’s how New Zealand law approaches that question, and what a capacity-based claim actually involves.

What Is Testamentary Capacity?

“Testamentary capacity” is the legal standard for whether someone was mentally capable of making a valid will at the time they signed it. New Zealand courts still apply the long-standing test from Banks v Goodfellow, which requires the will-maker to have understood:

• That they were making a will and what that means
• The general extent of the property they were disposing of
• Who might reasonably expect to benefit from their estate (spouse, children, dependants)
• That no delusion or disorder of the mind was distorting their decisions about who to include or exclude

A dementia diagnosis on its own doesn’t automatically mean a will is invalid. Capacity is assessed at the specific time the will was signed, and it can fluctuate — someone with early-stage dementia may still have had capacity during a lucid period.

Warning Signs Worth Investigating

• A will made or significantly changed shortly after a dementia or capacity-affecting diagnosis
• A sudden, unexplained change favouring one beneficiary over others who were previously treated equally
• The will-maker’s lawyer not obtaining a medical capacity assessment before the will was signed
• Evidence of confusion, memory loss, or disorientation around the time of signing
• A new will drafted with unusual involvement from the beneficiary who stands to gain

How These Claims Are Assessed

Capacity challenges typically draw on a combination of evidence: medical records from around the time the will was signed, notes from the solicitor who prepared it, and witness accounts of the will-maker’s presentation on the day. Courts give particular weight to whether the solicitor turned their mind to capacity at the time. These claims sit alongside, and are sometimes run in combination with, undue influence claims — see our guide to grounds for contesting a will in New Zealand for how these grounds fit together.

What Happens If a Will Is Successfully Challenged on Capacity Grounds

Where a court finds the will-maker lacked capacity, the will (or the relevant part of it) is treated as invalid. Depending on the circumstances, this can mean an earlier valid will is reinstated, or, if no earlier will exists, the estate is distributed under New Zealand’s intestacy rules.

Acting Early Matters

Capacity evidence gets harder to gather the longer a claim is delayed — medical records can be harder to access, and memories of witnesses fade. If you suspect a will doesn’t reflect a loved one’s true wishes because of their mental state at the time, getting an early legal opinion protects both the evidence and your options.

How Agile Legal Can Help

Capacity-based will challenges require careful handling of medical evidence and a clear-eyed view of how courts actually assess these claims. Agile Legal advises families considering a challenge, as well as executors defending one, as part of our will contest and estate litigation services. Before you start, confirm you have standing to contest the will, and see our guide on chances of successfully contesting a will in NZ for the broader picture.

Get in touch with our estate litigation team for a confidential assessment of your situation.

This article provides general information only and does not constitute legal advice. Every capacity claim depends on its specific facts — speak with a lawyer about your circumstances.

Frequently Asked Questions

Can a will be contested because the will-maker had dementia?

Yes, if the dementia meant they didn’t understand the nature of the will, the extent of their property, or who might reasonably expect to benefit, at the time it was signed.

Does a dementia diagnosis automatically invalidate a will?

No. Capacity is assessed at the specific time of signing, and someone with a diagnosis can still have had capacity during a lucid interval.

What evidence is used to challenge a will on capacity grounds?

Medical records from around the time of signing, the solicitor’s file notes on capacity, and witness accounts of the will-maker’s presentation on the day are the main sources of evidence.

What’s the difference between lack of capacity and undue influence?

Lack of capacity concerns whether the will-maker could understand what they were doing; undue influence concerns whether someone else improperly pressured their decisions. The two grounds can overlap and are sometimes argued together.