In 1901 Sir John Quick and Robert Garran published Annotated Constitution of the Australian Commonwealth. This book gives an historical introduction to the Constitution, its original text (before it was changed by referendum) and an extensive commentary on the political, historical and legal aspects of the Constitution. These commentaries are not law; they are the writers’ interpretation in 1901 of the Constitution.
Section 51(xxi) of the Australian Constitution gives the federal Parliament the power to make laws about marriage. It does not say what marriage is or who can get married. The details of how marriages are conducted in Australia are contained in the Marriage Act 1961.