TONY Abbott says he will "do the right thing" and attend his gay sister's wedding but could never support the marriage.

And his sister Christine Forster's engagement to partner Virginia Edwards will not dim his Government's fight against the country's first same-sex marriage laws in the ACT.

"If there's a ceremony of some kind, yes, I will be there with a present," the Prime Minister told 3AW of his sister's upcoming nuptials.

"I'll do the right thing."

But while wishing the couple well, he revealed he saw no way he could ever change his mind and support same-sex marriage.

"I am a traditionalist on this," he said.

"From time immemorial in every culture that's been known, marriage, or that kind of solemnised relationship, has been between a man and a woman."

The Abbott Government will challenge new ACT legislation, which will see the first gay weddings in December, arguing they are unconstitutional and clash with federal marriage laws.

Their passage came the same week Ms Forster revealed her engagement and spoke of her hope the couple may one day marry in their home state of NSW.

Despite his feelings, Mr Abbott said the High Court challenge was a legal one, not a moral one.

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