Opposition Leader Bill Shorten says a marriage equality bill would pass if it were presented in federal parliament this week.

Speaking at Melbourne's annual Midsumma gay pride march today, Mr Shorten called on the prime minister to go back to being the old Malcolm Turnbull who believed in marriage equality.

Revelers enjoy the day at the annual Misdsumma Pride March in St Kilda. Credit:Photo: Meredith O'Shea

Mr Shorten says about 95 per cent of Labor MPs, a sizeable number of Liberal MPs as well as the Greens and non-aligned members of parliament would all vote for gay marriage if they had the option.

As a parade of rainbow-clad men and women filtered in and out of view, Mr Shorten promised that marriage equality could happen under a Labor government without wasting $160 million on a plebiscite.