THE Premier, Barry O'Farrell, will allow Liberal MPs a conscience vote on a bill to legalise same-sex marriage, which a cross-party group of MPs will introduce this year.

Liberal, Nationals, Labor and Greens MPs have formed a working party to draw up a bill to allow gay and lesbian couples in NSW to marry, if legislation before Federal Parliament is voted down, as expected.

The group will follow the lead of other states, including Tasmania and South Australia, in pursuing same-sex marriage on a state basis.

Mr O'Farrell confirmed MPs would not be bound to vote on party lines, just as he did when a symbolic motion supporting same-sex marriage came before the upper house in May.