Property (Relationships) Act 1976, ss 2B, 2BA
In terms of working out the length of the relationship, a de facto relationship that begins before the marriage is treated as if it were part of the overall length of the marriage relationship.
Similarly, if a married couple were in a civil union before they got married or the other way round, the length of the relationship for the purposes of the Property (Relationships) Act is the total of both.