Prime Minister Tony Abbott’s sister Christine Forster has called for a bipartisan approach over moves to legalise gay marriage.

Labor leader Bill Shorten has introduced a marriage equality bill into the House of Representatives on Monday that will define marriage as a union “of two people” rather than a “man and woman”.

Ms Forster, who is engaged to a woman, said unless the bill received bipartisan support in Parliament it may not even be debated.

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“This has to be a cross party reform. It’s the only way it can successfully move ahead,” Ms Forster told the ABC.

“Labor’s insisting on putting up a private members’ bill but that private members’ bill will not give them ownership of this issue because it will not go anywhere.

“It’ll be introduced to Parliament, it’ll sit there, it won’t be debated, there won’t be a vote, it’ll just lapse.”

Ms Forster was one of the keynote speakers at a marriage equality rally in Sydney on Sunday afternoon that also included Deputy Opposition Leader Tanya Plibersek.

About 1000 people marched through the streets of Sydney ahead of the rally in Kings Cross.

Tanya Plibersek: “This is the last piece of unfinished business to full equality for gay men and lesbians in this country.” Photo: AAP

Ms Plibersek backed Ms Forster’s call for a bipartisan approach to the issue.

“This is the last piece of unfinished business to full equality for gay men and lesbians in this country,” Ms Plibersek told the rally.

“As one of our great prime minister’s [Gough Whitlam] said: ‘men and women of Australia – it’s time.'”

Ms Plibersek said that in 2012 a similar bill went before Parliament and about one third of MPs voted in favour of it, but predicted this time around the new bill would receive far greater support.

“We can only do this if it is multi-party and bipartisan. This has to be across the political spectrum,” Ms Plibersek said.

“Across the Australian community our citizens are demanding of our political leaders that this happens now.”

Ms Forster said all she wants is to have her relationship solemnised by the state, as it would be if she married a man.

“It doesn’t change the institution of marriage at all, the institution of marriage still remains being about two people committing to each other, hopefully for life, and that being legally recognised by our Government. That’s all that the change is,” she said.

However, Parliamentary Secretary to the Attorney-General, Senator Concetta Fierravanti-Wells said there was still strong support for “traditional” marriage in the community.

Ms Fierravanti-Wells said her view remained that marriage was between a man and a woman.

“I certainly have been proud to represent the views of those people who subscribe and continue to subscribe to marriage being between a man and a woman and that’s certainly been my position for a long, long time and will continue to be my position,” she said.

Ireland referendum ‘sends signal’ on reform

Ms Forster said if the gay marriage debate can receive bipartisan support, there was a strong chance Australia would follow Ireland’s lead in accepting a change in law.

Earlier this month Ireland overwhelmingly voted in favour of gay marriage at a referendum which attracted international headlines.

“The result in Ireland was undoubtedly a watershed,” Ms Forster said.

“It was a very decisive result, it was a wonderful result really.

“That’s sent the signal to anyone who might have been holding out against this potential reform that the tide has turned.”

Ms Forster called on both major parties to adopt a progressive approach to the issue.

“There is some politicking in this, there’s no doubt,” Ms Forster said.

“Behind the scenes, certainly within the Liberal Party, there’s been for a long time now people working behind the scenes to get to the position where this can come to a debate within the party room.”

Irish people celebrate outside Dublin Castle following the announcement of the result of the same-sex marriage referendum.

Australian gay marriage ‘inevitable’

Ms Forster said public support for gay marriage had been galvanised so strongly in recent weeks, she believed it would be accepted in Australia “sooner rather than later”.

“There is very much a feeling that this will be a free vote, a conscience vote, when the time comes for it to go to the Parliament and be voted on by all of Parliament,” she said.

“I do think it is inevitable and I’m very optimistic that we can get a result on this before the end of the year.

“I don’t think it’s in anyone’s interests to have this issue bubbling along when we come to an election year.

Ms Forster said it was “pretty clear on this one that the majority of Australians are in favour of this”.

“The latest polling was that 72 per cent of people are in favour and that’s a pretty significant majority and that’s a bigger majority than Ireland,” she said.

“As time goes on, as we move closer through this process, I’m sure that Federal Parliament will start to reflect that view which is held by the general public.”