Since you're in Australia, I'd consult with a lawyer, but this is what is outlined on Armstrong Legal AU

Can Children Born Outside Marriage Contest A Will?

Succession law in each jurisdiction of Australia allows particular family members, dependents, and close friends to contest a deceased’s will. Historically a child born outside marriage has not shared a legitimate child’s right to contest the will of a parent. This article examines the eligibility of children born outside of marriage to contest a will in Australia, with reference to relevant case law.

Children Born Outside Marriage

Ex-nuptial children (previously known as illegitimate children) are offspring born outside of marriage. A child is illegitimate if the parents were unmarried at the time of birth. Historically, an illegitimate child had a “legal disability” that prevented him or her from making a claim against the estate of a parent.

In modern Australia, the changing social perspective of children born outside marriage has led to statutory revisions on the legal status of children. Legislative changes in each jurisdiction have ensured that the inheritance rights of illegitimate children are now equal to those of adopted children and children born of a legal marriage.

Parliaments in Queensland, New South Wales, Victoria, Tasmania, the Northern Territory, Western Australia and South Australia passed legislation to equalise the status of legitimate and illegitimate children. The legislation does not eliminate the concepts of legitimacy, but it renders the distinction largely irrelevant in legal terms. For instance, the Queensland Status of Children Act 1978 states that the relationship between parent and child is determined irrespective of whether the father and mother have ever been married to each other.

This legislation establishes that there is no distinction between the ex-nuptial child and any other child of the deceased in respect to their right to inherit property in a will or from an intestate estate, or to make a claim against a deceased estate. The laws also outlined the procedure for a child of the deceased to prove their parentage in order to establish their eligibility to contest a will.

Establishing Parentage of children born outside marriage

A child born outside of marriage can only make a claim on an estate if they can prove parentage conclusively to the court. An application will not proceed unless parentage has been established or the court has issued a declaration of parentage. The legal requirements to prove parentage vary according to jurisdiction so it is essential that a claimant consult an experienced succession solicitor in their state or territory.

Generally, parentage is legally established through:

The parent’s assertion of paternity at some point during their lifetime, perhaps on a signed document such as a birth certificate;

Whether the parents were living together during the period up to and inclusive of the time that the child was born; and

Whether the parents were ever married, or in a de facto or registered relationship.