Thousands of people attended a rally in Sydney, Australia ahead of the introduction of a gay marriage bill.

Opposition leader Bill Shorten is expected to introduce his legislative proposal on Monday.

Deputy Labor leader Tanya Plibersek was among the politicians who spoke at the rally.

She told the cheering crowd that there was a high likelihood of passage if members of Australia's ruling coalition were allowed to vote as they choose on the issue.

“Bipartisanship is so very important for this issue to succeed,” she said. “I know there are other Liberals and Nationals who are supporters of marriage equality.”

Christine Forster, a local politician and the gay sister of Australian Prime Minister Tony Abbott, an opponent of marriage equality, introduced to the crowd her “beautiful fiance” Virginia Edwards.

“Why wouldn't I marry this beautiful woman as soon as I possibly could?” Forster said.

Lawmakers in 2012 rejected a similar bill.

Shorten announced his plans shortly after Ireland became the first nation to approve marriage equality through a popular referendum.

The Irish vote has put pressure on other governments to follow suit, especially in nations – such as Australia – where support for such unions runs high.