Mr Abbott has always been an avowed opponent of same-sex marriage despite the fact that his sister is in a long-term same-sex relationship and is a prominent activist for marriage equality.

While Mr Abbott's opposition to the issue hasn't changed, he appeared to concede defeat in parliament yesterday.

Speaking in response to a marriage equality bill to be introduced by opposition leader Bill Shorten, Abbott said this is an issue that must be "owned by the parliament."

"If our Parliament were to make a big decision on a matter such as this, I want it to be owned by the Parliament, and not by any particular party," Mr Abbott said.

It's now been reported that the PM is likely to allow his MPs to vote with their conscience on any same-sex marriage bills - a move that will be crucial to reform passing.