Australian Prime Minister Tony Abbott's gay sister Christine Foster has announced that she and her partner of six years are preparing to marry.

Foster, a Liberal councilor at the City of Sydney, and Virginia Edwards told New Idea magazine that they're preparing to get hitched.

“We got engaged when we were away with friends on a trip to Broken Hill,” the 49-year-old Foster said.

Foster added that Abbott and his wife Margie “will definitely be there.”

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 challenge a proposed bill which seeks to legalize gay nuptials in the Australian Capital Territory (ACT), which includes Canberra, the nation's capital.

“It is pretty clear under our constitution that it is the Commonwealth that has responsibility for the rules regarding marriage,” Abbott is quoted as telling reporters from the Australian Broadcasting Corporation (ABC). “We think it's important that there be a uniform approach to marriage throughout the Commonwealth and that's what we are going to do our best to ensure.”

The marriage bill is expected to pass this month and gay couples in the territory could marry as early as December.