It is poised to be a rainbow summer, with a slew of gay weddings on the cards once same-sex marriage finally becomes law in coming days.

But thousands of same-sex couples will find themselves already married under Australian law as soon as the Governor-General signs off on the legislation.

The bill currently before the Parliament erases the ban on recognition of same-sex marriages solemnised overseas, and will apply retrospectively.

The Equality Campaign estimates thousands of Australian couples have already tied the knot in countries such as Britain, the US, Canada and the Netherlands, where same-sex marriage has been legal since 2001.