The prime minister hailed it a "momentous social reform" as debate began in the lower house on a private bill passed by the Senate last week.

"It's time for us to get on with it," he told parliament on Monday.

"The Australian people have said yes to marriage equality, yes to fairness, yes to commitment, yes to love."

Mr Turnbull credited the coalition for initiating the change, labelling the postal survey - and the almost 80 per cent response rate - one of the most remarkable political events in his lifetime.

"To same-sex couples in Australia, you are equal, you are respected, you are loved," he said.

In a largely light-hearted speech, Mr Turnbull rejected claims allowing same-sex couples to marry was a threat to traditional marriage, including his own.

Australia would be a stronger society if more people were legally married and fewer were divorced, he said.

"If we believe two gay people are better off together than living alone comforted only by their respective cats, then why should we deprive that relationship of equal recognition?"

Mr Turnbull doesn't believe the bill before parliament - drafted by Liberal senator Dean Smith - threatened religious freedoms, however he expressed support for amendments which will reassure and "respect these fundamental rights".

"There is nothing in the bill which prevents anyone from maintaining or adhering to the teaching of their church on marriage or morality, but we must not fail to recognise that there is sincere heartfelt anxiety about the bill's impact on religious freedom," the prime minister said.

Labor leader Bill Shorten agreed a vote on the issue was "long overdue" and asked gay and lesbian Australians to forgive politicians for years of disappointment and delays.

Now was a chance to atone for the inaction and failures of the past, he said.

"Today belongs to all the LGBTQI Australians who have borne the burden of a long battle for equality," he told parliament.

"We seek your forgiveness, we salute your courage and we thank you for including us in your historic moment."

Nearly 80 MPs are slated to speak on the legislation over coming days, with a final vote expected before Thursday, when parliament is scheduled to rise for the year.

Veteran Liberal MP Warren Entsch, who helped write the legislation, kicked off proceedings with a stern warning to his colleagues who want to stall its passage.

Australians were sick of excuses and delays, he said, noting he would oppose any amendments that sought to unwind or remove any legal rights or protections against discrimination.

"This bill will take from no-one. It simply makes a nation a kinder and fairer place," Mr Entsch, wearing a rainbow tie, said.

"Delaying equality for every Australian, whether they be from Bundaberg or Fremantle, is simply not good enough."

Labor MP Chris Hayes was a surprise supporter of the bill, pointing out the postal survey had changed the landscape markedly since parliament last debated the issue in 2012.

"While I voted no, and I would hasten to add as did the majority of my electorate (64 per cent), I believe it would be hypocritical and disingenuous to have participated in this democratic process and not accept the outcome," he said.

He told parliament people with a genuine interest in religious freedoms should wait and address those issues separate to the marriage bill.

Senator Smith, Attorney-General George Brandis and cabinet minister Simon Birmingham were in the chamber to watch the start of the debate, as were Labor's Penny Wong and Louise Pratt.