A former Australian prime minister who was a vocal leader of the national campaign against gay marriage attended the same-sex wedding of his lesbian sister Friday and welcomed his sister-in-law into his family.

Tony Abbott, who was replaced as prime minister in 2015 in an internal government power struggle, joined his sister Christine Forster when she married her partner Virginia Flitcroft in a civil ceremony overlooking the Sydney Opera House.

"Great family occasion. Very happy for Chris and Virginia. I'm looking forward to having a new sister-in-law," Abbott, a former Roman Catholic seminarian, told reporters on his way to the wedding.

Abbott as national leader angered gay rights advocates and many lawmakers in his conservative government by deciding in 2015 to put the gay marriage issue to an extraordinary national vote. His explanation was to spare his government colleagues an ugly and divisive debate 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 percent of voting Australians who took part in a postal survey last year opted for change. Parliament overwhelming supported legislation lifting the ban in December.

While Abbott vehemently opposes gay marriage, he has long said he would attend his younger sister's wedding if the same-sex marriage ban was ever lifted.

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.

But she said on Friday shortly before the wedding that the couple looked forward to Abbott and his wife attending.

"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," Forster told Ten Network television.

Forster and Abbott were vocal advocates on opposite sides of the debate that peaked during the two months of the postal survey late last year.

Abbott was headed-butted in September by a man wearing a gay-marriage badge who has since pleaded guilty to assault but has yet to be sentenced.