UPDATE: PRIME Minister Tony Abbott says there will be a debate inside the Liberal partyroom about legalising same-sex marriage.

It comes as Opposition Leader Bill Shorten will introduce a private member’s bill on Monday to change the Marriage Act and allow same-sex marriage.

“It is an issue where decent people can differ,” the Mr Abbott said today.

“I’m sure at that time we’ll have a very full and frank and candid and decent debate.”

Mr Shorten has given formal notice that he will introduce the bill — seconded by Deputy Leader Tanya Plibersek — to amend the Act “to establish marriage equality”.

Mr Shorten said the Bill will not have the universal support of his colleagues and will “challenge the deeply held personal beliefs of MPs and Senators on both sides of politics.”

“I believe the time has well and truly come for the Parliament to debate marriage equality,” Mr Shorten said.

“Our current law excludes some individuals — and to me, that is unacceptable.

“It says to them: your relationships are not equally valued by the state, your love is less equal under the law,” Mr Shorten said.

media_camera The Labor bill.

Labor members are currently permitted to have a conscience vote on the issue despite Ms Plibersek calling for Labor MPs to be tied to a binding vote for gay marriage because it is part of the ALP platform.

The move is expected to put pressure on Mr Abbott — a strong opponent of same-sex marriage — to allow a free vote in the Liberal Party on the issue.

“For marriage equality to happen, Tony Abbott has to give his MPs a free vote,” Mr Shorten said.

“I know there are Coalition MPs who’d support marriage equality if Tony Abbott granted them a free vote.”

Mr Shorten’s attempt to change the Marriage Act is the third push to allow same-sex marriage in Australia.

Greens senator Sarah Hanson-Young and crossbench senator David Leyonhjelm both have private member’s bills before parliament on changing the Marriage Act.

The latest move comes just days after Ireland voted overwhelmingly in favour of allowing same-sex marriage in a historic referendum on Saturday, prompting a push for Australia’s law to change.