The opposition leader passionately told a packed Sydney event that he was determined to see Australia allow gay couples to marry.
He said he expected the Yes vote to prevail in the postal survey, but not because of politicians.
"It will be because Australia has said it's time to may make marriage equality a reality. I'm very optimistic," Mr Shorten told the rally.
The prime minister told like-minded Liberals and Nationals on Sunday it was a question of "fairness".
Addressing the NSW Liberals and Nationals for YES campaign launch in Sydney, Mr Turnbull said this change had already happened in many countries around the world.
"In anyone of those nations has the sky fallen in, has life as we know it ground to a halt, has traditional marriage been undermined? The answer is plainly no," he said.
