Mark Dreyfus, who was the attorney-general in the Albanese government's first term before being deposed by factional colleagues after May's election win, lauded the ambition of former Labor governments for pushing the boundaries of social and cultural change.
He urged his colleagues to continue a push for a republic and four-year electoral terms to bring the Commonwealth in line with all Australian states and territories rather than making voters hit the polls every three years.
His tone is in stark contrast to Prime Minister Anthony Albanese, who has walked away from further referendums flagged by Labor after failing to enshrine an Indigenous advisory body in the constitution.
Mr Dreyfus acknowledged constitutional change was hard but rejected the idea there needed to be bipartisanship for reforms to pass.
He pointed to misinformation and disinformation circulated about the voice to parliament, as well as a complex and disparate 'yes' campaign, "particularly when contrasted with the simplicity and discipline of the 'no' campaign", that made it harder to win the referendum.
However he acknowledged going to a referendum alone made it harder to succeed.
"The failure of the voice referendum was one of the greatest disappointments of my public life," Mr Dreyfus said in a speech about Labor and the constitution in Canberra on Thursday night.
He urged his colleagues not to give up on future reforms.
"It's time to renew the campaign to establish an Australian republic," he said.
"In my inaugural speech in the House of Representatives in 2008, I spoke of my hope that one day in that chamber I would vote for a Bill to amend the Constitution to establish an Australian republic - that has not come to pass.
"In this sense Labor's constitutional ambitions have been dashed, and badly. Early Labor figures would be mightily disappointed."
Four-year terms should be put to the people sooner rather than later, he added.
Mr Dreyfus also pointed to the ambitions of former Labor governments to push the boundaries without referendums that had resulted in landmark Australian reforms, and said he had "cautious optimism" they would be pursued again.
Bob Hawke and his attorney-general Gareth Evans passed major environmental conservation laws, throwing everything and the kitchen sink to ensure they were upheld by the High Court, he said.
"Again ... the Labor government's constitutional ambition was accepted and rewarded in time," Mr Dreyfus said.
The Albanese government was using the full stretch of its constitutional authority on housing reforms, he posited.
Quoting former Labor prime minister Gough Whitlam, he rubbished the suggestion it was immoral for a government to push the limits of its power, saying: "If there is a community need, the government has a duty to use whatever power is available to meet it."
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