Nationals senator Barry O'Sullivan says he is committed to bringing on a powerful commission of inquiry into the banking sector, predicting up to four MPs in the House of Representatives could cross the floor and support his bill.

Warning Prime Minister Malcolm Turnbull to "sit up and take note" if the Parliament backs the measure in defiance of the government, Senator O'Sullivan said the government's adoption of a conscience vote on same-sex marriage had opened up a new tactic for Coalition MPs who support taking on the nation's banks.

"The introduction of [Senator Dean Smith's bill to legalise same-sex marriage] into the Senate showed a new pathway for backbenchers to be able to pursue matters of importance to them and I'm just simply following along in his footsteps," Senator O'Sullivan told ABC radio on Monday morning.

"If both houses of the Parliament think this is a good thing to do and that is the decision, I think the Prime Minister has to sort of sit up and take note and support the parliamentary decision," he said.