Gay marriage would pass if introduced to parliament today, according to Opposition Leader Bill Shorten

A marriage equality bill would pass if it was presented in federal parliament this week, Labor leader Bill Shorten says.

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

Mr Shorten said 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.