In an article in the church newspaper Southern Cross, Peter Jensen said codifying gay unions under the Marriage Act would lead to the redefinition of "one of the indispensable foundations of community."
"Ensuring public honor of same-sex relationships by calling them marriages is an abuse of marriage itself,'' he wrote. "It imposes, through social engineering, a newly minted concept of marriage on a community that understands it in quite another way.
"This claim for a right to be married could open the way for other forms, such as polygamous marriages or perhaps even marriage between immediate family members," he warned.
"Ministers of the Gospel will find it increasingly difficult to teach Christian sexual ethics … since what they say will be contrary to what the state says," he said.
The prelate said he supports giving same-sex couples the economic rights of married people, however.
Alex Greenwich, of the gay lobby Australian Marriage Equality, said Jensen's "alarmist predictions" have not been borne out in nations that allow same-sex marriage.