The billboard, outside St Matthew-in-the-City, shows two model brides kissing atop a wedding cake, with the text "We don't care who's on top".
It comes as Labour MP Louisa Wall's bill on marriage equality is expected to pass its first reading in Parliament later today.
The progressive church has already thrown its support in behind the bill, and has countered claims by the bill's opponents that the law change will mean church ministers cannot refuse to perform same-sex marriages.
St-Matthew-in-the-City already performs civil unions.
"It should be of no surprise that St Matthew's with its long history of supporting the full inclusion of the LGBTQ community in both the church and society endorses the bill. Our question is why would any church not endorse two people making a loving commitment to each other?" Reverend Clay Nelson said.
"Through our billboard we are expressing our solidarity with the many Christians in many denominations who are calling on Members of Parliament to support Louisa Wall's bill."