PRIME Minister Tony Abbott says his objection to the ACT's decision to legalise gay marriage is a "legal one", not a moral one.

Mr Abbott confirmed on 3AW this morning that the Government would challenge the ACT Labor Government's Marriage Equality Same-Sex Bill, passed in the Legislative Assembly yesterday, on the grounds that it was inconsistent with the Commonwealth Marriage Act.

"The Attorney-General (George Brandis) has instructed the Commonwealth lawyers to challenge this legislation in the High Court because under our consistution, it is pretty clear that the Commonwealth is responsible for marriage and the regulation of marriage," Mr Abbott said.

Mr Abbott suggested same-sex couples "hold off" tying the knot in the territory as any marriages performed may become invalid if, as he expected, the High Court challenge was successful.

"(My objection) is purely a legal one. I think the constitution should be adhered to," he said.

Mr Abbott said his sister Christine, who became engaged to her long term partner Virginia this week, "chews his ear off" on the subject of gay marriage regularly.

But she was unlikely to change his mind.

"She's a terrific advocate," he said.

"If there is a ceremony of some kind, yes I'll be there, with a present. I'll do the right thing. But I am a traditionalist."

Under the ACT's law, same-sex couples from across Australia will be able to marry in the ACT by the end of the year.

Mr Abbott also staunchly denied climate change had caused the fires devastating NSW, despite the United Nations' climate chief Christiana Figueres determining there was "absolutely" a link.

"I think that official is talking through her hat," he said.

"We've have bad fires since the beginning of European settlement.

"Climate change is real ... and we should take strong action against it. But these fires are not an action of climate change."

Mr Abbott confirmed his commitment to selling Medibank Private in the interview with 3AW's Neil Mitchell, but would not say when the health insurer would be privatised.

The Prime Minister also defended the Government's decision to raise Australia's debt ceiling to $500 billion.

Mr Abbott said Australia was forecast to hit its peak debt of about $370 billion in two months' time and, with the peak now expected to rise "well over" $400 billion, the new limit was appropriate.

"The last thing we want it a crisis in this country like they've had in Washington," he said.

samantha.landy@news.com.au