Australian Prime Minister Tony Abbott says he'll attend his gay sister's wedding but remains opposed to marriage equality.

Christine Foster, a Liberal council at the City of Sydney, and Virginia Edwards became engaged just days before the Australian Capital Territory (ACT) became the first Australian jurisdiction to allow gay and lesbian couples to marry.

Foster and Edwards plan to marry in their home territory, New South Wales (NSW), which is also expected to debate the issue.

Foster, 49, has said that her brother and his wife Margie “will definitely” attend the wedding.

“If there's a ceremony of some kind, yes, I will be there with a present,” Abbott told 3AW. “I'll do the right thing.”

But Abbott added that he's a “traditionalist” on marriage.

“From time immemorial in every culture that's been known, a marriage, or that kind of solemnized relationship, has been between a man and a woman,” he said.

Abbott, whose conservative Liberal-National coalition swept into power on September 7 on a pledge to lower taxes, has steadfastly opposed marriage equality and has pledged to legally challenge ACT's marriage law.