A motion calling on federal Nationals MPs to be given a conscience vote on same sex marriage has been narrowly defeated at the party's NSW conference.

Federal leader Warren Truss argued such a move would breach a coalition promise to oppose gay marriage made before the last election.

The motion put by the Young Nationals was defeated by just 11 votes today.

Speaking against the motion Mr Truss said it was not just "about the issue of same sex marriage, it is also about honesty and integrity in government".

"The Nationals have always respected the right of their colleagues to cross the floor and vote differently if they feel that is important to them," Mr Truss told delegates in Bowral, in the Southern Highlands.