The Australian Senate wants all members of parliament to have a conscience vote on gay marriage.

Greens senator Sarah Hanson-Young won upper house support on Wednesday for a motion stating every person in federal parliament should be able to freely decide their position on marriage equality.

Senator Hanson-Young said it was time for Prime Minister Tony Abbott, who has so far refused a conscience vote, to catch up.

But Attorney-General George Brandis said the motion was redundant for Liberal and National party members because "every vote is a conscience vote".

"All members are always at liberty to vote at variance from their party's agreed position," he told parliament.

The coalition doesn't suspend members for crossing the floor, however if a minister votes against the party they must retire from their frontbench position.