The assumption is that when a couple is married, or in a de facto relationship that they will remain faithful to each other for the entirety of the time they are together. When that is not the case, and one partner discovers that the other has cheated on them, then their next move tends to be to call their family lawyer at to start divorce proceedings.
Invariably they will tell their divorce lawyer that the reason for the divorce is infidelity, however, since 1975, getting divorced for that reason has not been a legal possibility. This is due to the fact that the Family Law Act of that year entered into Australian law the principle of no-fault divorce.
Prior to 1975, if you had wanted a divorce, you would have had to sit down with your divorce lawyer and basically build a case against your spouse to show that they were to blame for the marriage ending. Obviously, adultery, if proven or admitted by the other spouse, would indeed show that they were to blame, and the divorce would be granted on that basis.