A postal vote on same-sex marriage would likely be invalid without legislation and struck down in the High Court, according to legal advice obtained by marriage equality advocates.

In the first detailed legal opinion to be presented on a same-sex marriage postal vote, lawyers argued the Turnbull government could not circumvent Parliament to enact a postal plebiscite.

More than 54,000 new voters have enrolled in the past two weeks. The roll closes Thursday.

The advice, prepared by Ron Merkel QC and Christopher Tran, and seen by Fairfax Media, suggested the High Court would likely rule a postal vote invalid unless authorised by a specific law.

Same-sex marriage campaigners have seized the advice and committed to a legal challenge if Prime Minister Malcolm Turnbull goes ahead with the idea, which has been gaining traction among Coalition MPs.