For a de facto relationship to be governed by South Australian law, it must come under the definition of a 'domestic partnership'. Domestic partnerships include opposite-sex relationships, same-sex relationships, and companion relationships. This section deals with opposite-sex relationships.

The definition of a domestic partnership for most areas of South Australian law is set out in the Family Relationships Act 1975. Under this Act, a 'domestic partnership' is two adults in a 'close personal relationship' (that is, living together as a couple on a genuine domestic basis) where the partners have been together for at least three years, or during four years for periods totalling three years, or have had a child together.

However, if the relationship was for less than three years and there is no child of the relationship, either of the former partners may seek a declaration from the Court that they were domestic partners, on the basis that they were living together in a close personal relationship and that the interests of justice require that a declaration be made. This application is made under the Family Relationships Act 1975, which sets out what the court must consider when making its decision. Note that this process cannot be used to enable an application to court to resolve a property dispute under the Domestic Partners Property Act 1996, which requires that the relationship must have existed for at least three years, or that there is a child of the relationship, before an application to divide property can be made.

Domestic partnerships do not include marriage relationships or situations where care is provided by one person to the other for fee or reward.

For most legal purposes, partners do not have to take any formal step to have the relationship recognised. Once the criteria are met, the relationship is recognised automatically. If there is doubt, the court can be asked to make a declaration as to whether two people were, on a given date, domestic partners.

For some legal purposes, a formal declaration from a court is required before the relationship can be recognised as a domestic partnership.

In general, de facto relationships are a matter for State Governments because they are outside the Constitutional powers of the Commonwealth Parliament. Some de facto relationships are recognised by certain Commonwealth Acts for certain purposes, such as migration and pensions.

However, the Family Law Act 1975 (Cth) treats all children the same, whether their parents are married or not. Under South Australian State law , the fact that a child's parents are unmarried is irrelevant if the child wants to dispute a deceased parent's will.