Altering the definition of marriage to allow same-sex couples to wed would be the equivalent of defining sheep stations as lighthouses, a Senate committee has been told.
The first 70 of what are expected to be thousands of submissions to an inquiry into a Greens bill to legalise gay marriage have been publicly released.
Newcastle University senior law lecturer Neil Foster told the inquiry that any attempt by Parliament to change the nature of marriage by allowing weddings between same-sex partners would be unconstitutional.
''The constitutional power over lighthouses, for example, cannot be used to regulate sheep stations by defining a sheep station as a lighthouse,'' Mr Foster said in his submission.
''The word 'marriage' as used in 1901 and as consistently used in the law ever since is not apt to describe a legal union between two persons of the same gender.''
Mr Foster, who made the submission in a private capacity and not on behalf of the university, said there were also social policy arguments against gay marriage.
He said the important features of family life would be undermined by the attempt to give the label marriage to relationships that were ''by their very nature, not designed to promote the birth of children who will be cared for by their biological parents, and which in fact share very few other characteristics of the usual institution - not being essentially aimed at fidelity, and not seriously being intended for life''.
Professor Andrew Lynch, Professor George Williams and Emily Burke, of the Gilbert and Tobin Centre of Public Law at the University of Sydney, said Parliament probably had the power to legislate for same-sex marriage.
''On balance, it is arguable, though uncertain, the High Court at the present times will find that Commonwealth possesses legislative power to permit same-sex unions under section 51 (xxi),'' they wrote.
The submission argued that the current prohibition on same- sex marriage laws was discriminatory.
''Second, recognising same-sex marriage would better reflect Australia's approach to balancing respect for religious freedom with the separation of church and state.''
Amnesty International also argued that same-sex civil marriage was an issue in which fundamental human right issues were at stake.
''Amnesty International supports all initiatives to end discrimination in civil marriage laws based on sexual orientation or gender identity,'' the human rights group said.
Family Council of Victoria secretary Bill Muehlenberg said same-sex marriage was a ''bad idea'' and would be harmful to society.
''It is bad for society, bad for marriage and bad for children,'' Mr Muehlenberg said.
''The concept is oxymoronic, and it confers no benefits to society.''
The Senate Legal and Constitutional Affairs Legislation Committee has warned that staff and resource limitations will prevent it from publishing all of the submissions made to the inquiry, especially form letters,
The committee has promised to publish a selection of the submissions which are representative of a broad range of views.