Christine Forster says brother Tony Abbott is "just plain wrong" on same-sex marriage.

This morning, former Prime Minister and current Liberal backbencher Tony Abbott urged his fellow Australians to vote ‘no’ on a postal plebiscite vote to legalise same-sex marriage.

For City of Sydney Councillor Christine Forster, Tony’s words hit close to home.

The sister of the former PM, Christine been engaged to her partner Virginia Edwards since 2013. The pair has plans to marry as soon as their union becomes legal.

So when Tony Abbott urged Australians to vote no "if you don't like same-sex marriage...if you're worried about religious freedom and freedom of speech... and if you don’t like political correctness", she had something to say.

Appearing on The Project, Christine said her brother was "just plain wrong" when it came to the issue of legalising same-sex marriage.

"They’re all pretty flimsy arguments from Tony, frankly," she told the show's panel.

"He is wrong. He’s just plain wrong.

"Whether or not I marry my partner, Virginia, which we plan to do, it has no impact on Tony."

When asked if she felt Tony's comments were directed at her personally, Christine said she didn't think her brother "thought of it in those terms".

"But as a gay woman who is in a relationship, who wants to get married, and Tony is well aware of our plans to get married, it's not a really nice thing to hear somebody belittling our plans," she said.

"Particularly when he's your brother and you love him. It's unpleasant."

LISTEN: Mamamia Out Loud has a message for politicians about the same sex marriage plebiscite. (Post continues below).

It's a sentiment echoed in a series of tweets the Councillor fired off in response to her brother's statements to the media.

"If you value mutual respect: vote yes. If you want all Australians to be equal: vote yes. If you believe in free speech: vote yes," she tweeted.

She added that a 'no' vote would make the relationships of others "second rate".

If you want the person you love to be in every sense a part of your family: vote yes #marriageequality #auspol

— Christine Forster (@resourcefultype) August 9, 2017

If you don't believe your relationships (or anyone else's) are second rate: vote yes #marriageequality #auspol

— Christine Forster (@resourcefultype) August 9, 2017

Despite Tony's close link to someone who wishes to marry their partner, but who is legally unable to do so, Christine believes "nothing will sway him" in his views.

"He's been put on the spot by his own kids... his kids are definitely in favour of marriage equality and that hasn't persuaded him," she told The Project.

"He takes a very traditional view...and I know that nothing will sway him on that so I don't try and convince him."

At the end of the day, Christine said, it comes down to mutual respect.

"This is about all Australians being equal before the law... and if Tony wants to make it about free speech, then it is about free speech.

"It's about my right to be able to stand in front of my family and friends and say 'I do' to Virginia."

Listen to Christine tell the Just Between Us podcast about coming out later in life