The Marriage Act of 1961 is also involved with the Australian Constitution Section 51 of the Australian Constitution detailing what Marriage actually means under Australian Constitutional Law.
High Court of Australia
The interpretations of the High Court of Australia and terms of reference of the Marriage Act of 1961 and the Australian Constitution section 51. Now going through the Marriage Act and its various ammendments is a time consuming project. But the Parliamentary Paper did set out was a Marriage. And that is a Voluntary Union between One Man and One Woman at exclusion to all else. If you are sourcing reference to what is termed by Law I suggest you actually search the exact meaning.