In making a will, a person must have the testamentary capacity (mental capacity) to know what he or she is doing. They must know that, by their actions, they are making a will which will distribute their property after death. A court or tribunal may limit a person's will making capacity - for example, where a protection order is made under the Aged and Infirm Persons Property Act 1940 (SA) or where directions are made in relation to a protected person under the Guardianship and Administration Act 1993 (SA) [s 56].

Where there is any doubt about a person's mental capacity great care must be taken when making a will as it may be subject to challenge. It is always wise in such a case to have the will prepared professionally, and to have it witnessed by the person's doctor who could testify later as to the person's mental capacity.

Anyone over 18 years, with the required mental capacity, can make a will in South Australia [Wills Act 1936 (SA) s 5(1)]. People under 18 years can make a will if they are or have been married or with the permission of the Supreme Court. The Supreme Court may also authorise a will or an alteration to be made where a person lacks testamentary capacity (mental capacity) [Wills Act 1936 (SA) s 7].