More than 20,000 people gathered in Sydney to campaign ahead of a non-binding postal vote on changing Australia's marriage act.
Mr Turnbull made a surprise appearance and speech at the launch of the New South Wales Yes campaign.
Opposition leader Bill Shorten then addressed the crowd at the main rally.
The non-binding vote to gauge support for changing Australia's Marriage Act will be sent out from 12 September, with a result expected in November.
