Tony Abbott, the former Australian Prime Minister and vocal opponent of gay marriage, has celebrated his sister’s same-sex wedding.

Christine Forster married Virgina Flitcroft in a civil ceremony overlooking the Sydney Opera House, less than three months after the public rejected her brother’s campaign against such weddings in a referendum last year.

It was “great family occasion”, declared Mr Abbott. “Very happy for Chris and Virginia. I’m looking forward to having a new sister-in-law.”

His welcoming words were a marked change in tone from the divisive referendum campaign, during which the former Liberal Party leader warned same-sex marriage threatened “religious freedom” and accused pro-change campaigners of “bullying and hate speech”.

“It’s time to say that political correctness has got completely out of hand and to vote ‘no’ to stop it in its tracks,” he wrote in the Sydney Morning Herald.

Mr Abbott had been a vocal opponent of gay marriage for years before the referendum and was one of the most high-profile campaigners against change.

As an opposition front-bencher in 2008, he said: “However deeply affectionate or long lasting it may be, the relationship between two people of the same sex cannot be a marriage because a marriage, by definition, is between a man and a woman.”

Tony Abbott was one of the most high-profile campaigners against gay marriage (EPA)

In September last year, Mr Abbott was headed-butted by a man wearing a gay-marriage badge as the referendum campaign turned ugly. His attacker has since pleaded guilty to assault but has yet to be sentenced.

A former Roman Catholic seminarian, Mr Abbott angered gay rights advocates and many in his conservative government by deciding in 2015 to put the gay marriage issue to an extraordinary national vote.

He said it would avoid a divisive debate between government colleagues to resolve the issue.

Critics suspected he was betting that Australia’s recent history of resisting change in referendums would maintain the ban on same-sex marriage. But 62 per cent of voting Australians who took part in last year’s postal survey opted for change. Parliament overwhelmingly supported legislation lifting the ban in December.

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While Mr Abbott vehemently opposed gay marriage, he had long said he would attend his younger sister’s wedding if the same-sex marriage ban was ever lifted.

Ms Forster, an elected member of the Sydney municipal council, recently told reporters that her brother’s promise to come to her wedding was presumptuous because he had yet to receive an invitation. She had been vocal campaigner for legalisation of gay marriage and publicly clashed with her brother ahead of the referendum.

But Ms Forster and her partner, who had been engaged for four years, said shortly before Friday’s ceremony that they were looking forward to Mr Abbott and his wife attending. The former PM joined several prominent figures from the “yes” campaign at the wedding.

Mr Forster said: “He’s been fabulous. In fact, he is the first person to ring us this morning to check in that everything was going smoothly and that there hadn’t been any last-minute hitches.”