Australia's Catholic church is threatening to fire teachers, nurses and other employees who marry their same-sex partner if gay marriage is legalised, in a dramatic move led by the country's most senior Catholic.

Archbishop of Melbourne Denis Hart, speaking exclusively to Fairfax Media, pointedly warned the church's 180,000 employees they were expected to uphold its teachings "totally", and defiance would be treated "very seriously".

"I would be very emphatic that our schools, our parishes exist to teach a Catholic view of marriage," he said. "Any words or actions which work contrary to that would be viewed very seriously.

"Our teachers, our parish employees are expected totally to uphold the Catholic faith and what we believe about marriage. People have to see in words and in example that our teaching of marriage is underlined.