A prominent US politician has lashed Australia's stance on gay rights as "increasingly" behind the rest of the developed world as Sydney prepares to celebrate the gay and lesbian Mardis Gras on Saturday.

Barney Frank,who was a US congressman for 30 years and is a long-time campaigner for same-sex rights, also described Prime Minister Malcolm Turnbull's support for a plebiscite on same-sex marriage as laughable.

More than 20 countries allow same-sex marriage, including Canada, the United Kingdom, United States, France, New Zealand, Ireland and South Africa. Mr Frank married his partner Jim Ready in 2012.

Mr Frank said "increasingly" Australia's stance on same-sex rights was "behind much of the developed world".