The Opposition has confirmed it won’t back the government’s proposed plebiscite on gay marriage.

Surrounded by families of same-sex couples who had gathered at Parliament House to celebrate the decision, Mr Shorten revealed the Labor caucus had voted to oppose the proposed public vote unanimously.

“Upon my recommendation and that of our shadow attorney-general and our shadow minister standing behind me, the caucus unanimously resolved to oppose the marriage plebiscite,” he said.

“The Labor Party will in parliament oppose Malcolm Turnbull’s expensive, divisive plebiscite.”

Mr Shorten said he had reached his decision in consultation with professionals and families who believe the plebiscite would directly affect them.

“We’ve listened to the experts, to mental health professionals, and the experts have unequivocally explained to Labor that the plebiscite could cause harm to gay and lesbian people, particularly, but not exclusively, young people,” he said.

“Having met these families, having listened to these stories, I could not in good conscience recommend the Labor Party that we support the plebiscite about marriage equality.”

Mr Shorten labelled the government’s proposal a “shocking waste of $200 million plus”.

“What is the point of compelling 15 million of our fellow Australians to vote when Malcolm Turnbull couldn’t guarantee his members would comply with the vote,” he said.

The Labor leader said he would continue to press for a free vote in the parliament.

“We could make marriage equality a reality today by having a free vote in the parliament and that is what should be done,” he said.

“This is the quickest, cheapest, least harmful and most certain path and we will be pressing our case in days and weeks to come.”

Labor’s opposition means enabling legislation for the plebiscite, to be debated in parliament this week, would not pass through the Senate.

Before Labor formalised its opposition to the public vote, Attorney-General George Brandis released draft amendments to the Marriage Act that could be enabled by a plebiscite late on Monday night.

The changes include redefining marriage as between “two people” rather than “a man and a woman” and allowing ministers of religion and non-religious celebrants to refuse to marry a same-sex couple.

Prime Minister Malcolm Turnbull and other senior coalition MPs have consistently said a plebiscite was the only way forward for marriage equality in Australia under the current term of government.

Government figures have this week refused questions over whether they have a Plan B if the plebiscite was voted down.

Though Labor has pledged to push for a parliamentary vote on the issue, senior Government ministers have made it clear this won’t happen.

Earlier on Tuesday, Nationals MP Andrew Broad threatened to withdraw his support for the Turnbull government if a free vote went ahead.

With AAP