Bill Shorten has accused the prime minister of treating Kevin Rudd in a "shabby fashion" for "petty politics" after he refused to endorse him for the United Nations top job.
Australia won't be nominating Mr Rudd for secretary-general of the UN even though he claims Malcolm Turnbull had told him Australia would be "mad" not to support him.
The opposition leader echoed a comment by his senior minister, Anthony Albanese, labelling Mr Turnbull's actions "pathetic".
"He has squibbed the chance to be a leader for all Australians and all he's trying to do is paper over the divisions in the Liberal Party," Mr Shorten told reporters in Darwin on Saturday.
"And in the meantime, a distinguished Australian has been treated in a very shabby fashion for nothing more than, I think, petty politics.
"Malcolm Turnbull's actions are pathetic. They're disappointing."
Foreign Minister Julie Bishop and Attorney-General George Brandis are understood to have supported Mr Rudd's nomination, but several conservative ministers including Immigration Minister Peter Dutton and Treasurer Scott Morrison spoke against it.
Mr Rudd took to Twitter to thank Ms Bishop for her support, saying "unfortunately" the prime minister had disagreed.
The prime minister said on Friday the government would not back the former Labor prime minister because he didn't believe he was suited for the role but refused to give details.
Late on Friday Mr Rudd released three letters he had sent to Mr Turnbull in which he refers to the prime minister's apparent support.
In the second letter, Mr Rudd said he was "shocked" the prime minister told him he no longer supported his bid after "You had always said to me that the Australian government would be `mad' not to support my candidature."
He wrote another letter this week asking for a personal meeting to "simply ask for the right to be heard".
After Mr Turnbull announced his decision, Mr Rudd said it was a "pity" the government did not support him and that the prime minister would not explain the decision in person, even though they were both in Sydney at the same time.
"It would have reflected well on what our nation can offer to the world - as a middle power with relationships across the world, including the developing world, smaller states, the Commonwealth, our Pacific Island friends and of course our partners in Asia," Mr Rudd said in a statement.
But Deputy Prime Minister Barnaby Joyce said a person should not be backed for a top role just because he was Australian.
He told ABC television it was Labor who had poor character assessments of Mr Rudd.
Mr Turnbull said the decision had nothing to do with Mr Rudd being a former Labor leader, pointing to Kim Beazley's appointment as US ambassador.
It is possible Australia could back former New Zealand prime minister Helen Clark for the UN role.