Australia's most senior churchmen, including the Anglican and Catholic archbishops of Sydney, have made a list-ditch plea to political leaders to salvage religious exemptions in the same-sex marriage bill.

Writing to Prime Minister Malcolm Turnbull and Opposition Leader Bill Shorten, as many as 30 church leaders asked for reconsideration of a suite of amendments first proposed by Liberal senator James Paterson.

The amendments - including exemptions for religious charities and legal protections for parents who want to pull their children out of particular classes - were already defeated this week in the Senate, which passed the bill unamended on Wednesday.

However the proposed revisions are expected to be prosecuted again in the House of Representatives next week, and the letter represents a last-minute attempt to shift the position of the party leaders and their MPs.