Telling fellow MPs the "opportunity of a generation" was before them, Mr Shorten also drew on former prime minister Gough Whitlam's famous line, "it's time".

"Let this law reflect the nation we all want to see in the mirror," the Labor leader said. "Generous, smart, modern, diverse and above all, equal."

Mr Shorten's wife, Chloe, watched from the public galleries as he delivered a short speech, introducing a bill that would change the definition of marriage to "the union of two people".

Labor senators Penny Wong and Katy Gallagher and ACT chief minister Andrew Barr were also in the chamber.

But despite the strong Labor showing, the Coalition side of the House of Representatives was almost empty, with less than a handful of government MPs in attendance.

There is no obligation on the Coalition to watch a private member's bill be introduced, other than the required number of duty MPs.

Mr Shorten's move to bring forward his bill has angered Liberal MPs, who have accused the Labor leader of grandstanding and politicising the issue.

Same-sex marriage supporters in the Coalition say they are working to bring same-sex marriage before the Parliament in the second half of the year.

Mr Shorten introduced his bill despite Labor failing to get a Liberal MP to co-sponsor it.

While Coalition MP Warren Entsch has agreed to work with Mr Shorten on a bi-partisan process, he said he would only do so after the budget sittings.

On Monday, while visiting a Harvey Norman shop to promote the budget, Mr Abbott said his "priority is getting the budget measures through the Parliament".

"I accept that same-sex marriage is a significant issue," he said.

"But frankly, this government's absolute fundamental priority in the budget session of Parliament is to get the most urgent budget measures through.

"And by far the most urgent budget measure is the small business budget boost."

Mr Abbott then stressed that not just him, but "every single one of us on the Coalition side" was focused on the budget for the next few weeks.

"We don't have to do everything immediately."

When asked if he thought most Australians supported same-sex marriage, Mr Abbott replied: "Let's see where the community debate goes."

The most commonly cited opinion poll was done by the Liberal Party's own pollster Crosby Textor last year for Australian Marriage Equality. It found 72 per cent of Australians back same-sex marriage.

Australian Marriage Equality national director Rodney Croome said it was "impossible to stop the momentum for marriage equality in Australia".

"The Prime Minister may want to prioritise other issues and that is his right," Mr Croome told reporters in Canberra.

"But there is a conversation happening in the Australian public that will not be stopped."

He said that in recent days, about 100 corporations had been in touch with Australian Marriage Equality to express their support for same-sex marriage.

In his parliamentary speech, Mr Shorten described his bill as something that seemed like a small gesture but one that "means so much to so many".

"To same-sex couples, we offer the right to celebrate your love with the public measure of devotion: marriage."

Mr Shorten noted that two out of five young gay Australians had thought about self-harm or suicide.

"We say to all young gay people: we are proud of you, for who you are. You belong."

Mr Shorten's bill is now the third same-sex marriage bill before Parliament.

Both the Greens and Liberal Democrat David Leyonhjelm have similar bills before the Senate.

If Liberal MPs had a free vote on the issue, marriage equality campaigners estimate that same-marriage legislation almost has a majority in both houses.

Greens leader Richard Di Natale said the Parliament was "on the verge of achieving a profound change in the Australian community."

But he said Labor's bill was only "symbolic" and would not come to a vote.

"If we can get a co-sponsored bill by all sides of politics, we're as good as done."

Same-sex marriage