A former Australian prime minister who was a leader of the national campaign against gay marriage has attended the same-sex marriage of his lesbian sister in Sydney.

Tony Abbott joined his sister Christine Forster when she married her partner Virginia Edwards in a civil ceremony overlooking the Sydney Opera House on Friday.

Abbott told reporters it was a "great family occasion" and that he was "looking forward to having a new sister-in-law."

Almost 62 percent of voting Australians who took part in a postal survey last year opted for marriage equality and Parliament in December overwhelming supported legislation lifting the same-sex marriage ban.

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