All orders cease when the child turns 18 years, marries, enters into a de facto relationship or is adopted by another person [Family Law Act 1975 (Cth) ss 65H(2) and 65J(2)].
Where the child is adopted by a step parent, parenting orders are not altered unless the Family Law Court approved the adoption, see FAMILY RELATIONSHIPS - Adoption.
An order can be varied at any time, but unless there has been a significant change in the circumstances of either of the parents or the child, the court is generally reluctant to change existing orders except by consent of the parties.