Two separate bills, from the Green and Labour parties, were tabled in parliament on 18 June, along with a standing committee report which made no recommendation to legislators. Most submitters supported changing the marriage laws to include same-sex couples.
In letters read out at church services during the weekend, Greek Orthodox leaders encouraged parishioners to write to their legislators, while Catholic church leaders said "legislating for same-sex marriage will change the meaning of marriage for everyone."
Anglicans urged opposition to the redefinition of marriage.
"It is a contemporary tragedy that marriage is so little understood or honored," the letter from Sydney Anglican Archbishop Peter Jensen said.
But Perth Anglican Archbishop Roger Herft said the pulpit was an inappropriate platform to campaign, as it was not the role of the Church to have its members lobby politicians.
Sydney Baptist pastor Mike Hercock said the letters were fear-mongering, and urged Christians to make up their own minds. "They are persecuting their own who happen not to toe the party line," his statement said.
In a submission to the standing committee, Catholic Archbishop George Pell said only couples designed to be procreative should marry.
However, committee chair Graham Perrett, who is Roman Catholic, wrote in the committee report that passage of the legislation, which will not oblige clergy to conduct same sex marriages, will ensure equal rights in Australian society.
"Catholics in Australia don't all necessarily speak with the one voice of Archbishop George Pell," he later told Ten News.
A vote on the bills is likely later this year but Perrett has said he believes the majority of legislators don't share his views.
Supporters may stall the vote should Opposition Liberal Party leader Tony Abbott, who opposes the legislation, continues to deny his party a free vote to prevent legislative change.
Prime Minister Julia Gillard will permit Labor a free vote, while personally opposing the legislation.