When a person dies there are two ways their estate can be distributed - either pursuant to a will, or where there is no will, according to what are called the rules of intestacy. The rules of intestacy are established by the Wills, Probate and Administration Act 1898 (NSW) and are strictly applied. This means there is little recourse for those not covered by the rules, which distribute the estate to family members. If there are no family members, the estate will pass to the state. If this occurs, you can apply to the state for a share in the assets, but this will be an administrative matter judged on the merits of your case.
If a will exists, however, it may be possible to contest the distribution of the estate before a court. If you wish to do so, your best chance for a share in the estate is to contest that the will is not in fact valid.
To satisfy the court that the will is invalid, you will need to prove either: the document being relied upon was not the last will written; parts of the will were changed or added after it was signed; undue influence was exerted upon the will-maker; the tendered will was cancelled or revoked; or the deceased did not have the capacity to make a will.
To successfully contend the will is invalid for reasons of undue influence, you would have to prove that undue influence was exerted by someone who benefits from the will, possibly substantially. Additionally - because the term 'undue influence' has a strict legal definition - you would need to provide evidence that the person benefiting from the will was in a position whereby they overpowered both the judgment and the true wishes of the will-maker. This would probably mean you would need to present evidence from witnesses who saw the beneficiary exert power over the deceased.
To contend the will is invalid for other reasons would also, in many instances, require that witnesses be called and documents tendered. It is not sufficient that the person contesting the will state in court that, for example, the deceased said they were to receive certain assets.
Evidence of invalidity that is discernible from the face of the will itself (and therefore easily evinced) includes the lack of a testator's signature manifesting an intention to give force to the will. A signature with this force is usually to be found at the end of the will, although signatures elsewhere may appear to have the requisite intention.
This type of overt evidence of invalidity also includes a document that does not have signatures of two witnesses. It is worth noting that these witnesses must sign in the presence of the will-maker for the will to be valid. The only evidence that this has occurred, however, and which is discernible from the document itself, is an attestation clause to that effect. In the absence of this type of attestation clause, however, evidence (testimony or an affidavit) from a witness could have significant weight.
Another way you could contest the validity of a will is to assert the deceased did not have the mental capacity at the time they signed the document. The lack of capacity could be for one of three reasons: the age or illness of the will-maker; the testator's mental handicap; or the testator's mental illness. In all these instances, you will have to show that either the testator: did not know what a will was; did not understand what assets were being disposed of (including the value or amount of the assets); or were not able to judge morally who should benefit and how.
In each of these instances, evidence will need to be led to prove that capacity was lacking, such as a statutory declaration, affidavit or testimony from a doctor or other carer. Conversely, however, any existing notes made by the solicitor who drafted the will could be of assistance to the defendant if the notes demonstrate the testator exhibited capacity.
Additionally, one could contest the validity of a will on the basis that it has later been revoked or cancelled. A will can be revoked by a variety of actions on the part of the will-maker, but the most common act of revocation is marriage. A marriage subsequent to the signing of the will revokes the will unless the will was made in contemplation of that particular marriage, or the will itself states it is made in contemplation of a marriage.
Finally, the validity of a will can be contested on the basis that the will is written in English and the testator is not sufficiently competent in English to understand it. If there is evidence that the will was explained to the will-maker, however, then the defendant may be able to prove the will to be valid. This would require either a translation of the will, annexed to the English language version, or an attestation clause or statutory declaration stating that an interpreter translated the will for the testator.
If you believe you have evidence to support any of these reasons for invalidity, or indeed, someone is contesting a will you have benefited from, you should seek advice from a solicitor. Taking the matter to court will take time and money, but it may take even longer and require even more funding if you don't seek advice - and worst of all, you could loose your case. Most importantly, however, you should ensure you get a solicitor to help you draft your own will to ensure your wishes are eventually carried out to the letter.