The definition of a de facto relationship is found in s.4AA of the Family Law Act. The Act sets out a number of factors for the Court to consider when determining if the persons are in a de facto relationship.
a. the duration of the relationship;
b. the nature and extent of their common residence;
c. whether a sexual relationship exists;
d. the degree of financial dependence or interdependence, and any arrangements for the financial support, between them;
5. the ownership, use and acquisition of their property;
6. the degree of mutual commitment to a shared life;
7. whether the relationship is or was registered under a prescribed law of the State or Territory as a prescribed kind of relationship;
8. the care and support of children; and
9. the reputation and public aspects of the relationship.
No particular finding in relation to any circumstances are necessary in deciding whether the persons have a de facto relationship pursuant to s.4AA(3) of the Act.
A Court determining whether a de facto relationship exists is entitled to have regard to such matters, and to attach such weight to any matter, as may seem appropriate to the Court in the circumstances of the case, pursuant to s.4AA(4) of the Act.
In the Federal Magistrates Court case Aitken v Deakin  FMCAfam 35, McGuire FM said:
The concept of separation in both a legal and emotional sense is sometimes one difficult to isolate in time and definition. In most cases the point of time is determined by physical separation. There is public recognition and mutual acknowledgement of the separation often emphasised by manifestations of behaviour and statement consistent with the breakdown of the relationship. However, in other factual situations such manifestations are not so obvious. The fact is that the parties in a relationship do not always separate in dramatic and defined circumstances. It can be a gradual process towards physical separation. The law assists in requiring the development by one of the parties of a definitive intention to separate. That intention must be communicated to the other party in terms without ambiguity or conditions attached. There would normally be a requirement for a contemporaneous change in the nature of the relationship such as cessation of sexual relations or public socialising as a couple.
If you want to know more about the law applying to de facto relationships contact Swan Family Lawyers on 8227 1970 or firstname.lastname@example.org