A self-described straight crocodile hunter from Australia's socially conservative north will cut short a United Nations assignment in New York to lead reform in his homeland if Australians vote to legalize gay marriage.

Conservative government lawmaker Warren Entsch says he will present to Parliament a bill to allow same-sex couples to marry if a national postal survey reveals on Wednesday that most respondents favor marriage equality.

Rights advocates have lauded the 67-year-old as an unlikely leading agitator in conservative politics for gays achieving equality.

Entsch says with opinion polls in recent years showing most Australians support gay marriage, no one should accept the result of the postal survey if the answer is "no."