Prime Minister Malcolm Turnbull has criticised Kevin Rudd for releasing private correspondence relating to the former prime minister's attempt to become secretary-general of the United Nations.

Key points: Mr Turnbull says releasing the letters "says a lot about Mr Rudd"

Mr Turnbull says releasing the letters "says a lot about Mr Rudd" Mr Rudd released letters claiming Mr Turnbull supported his candidature for the UN job

Mr Rudd released letters claiming Mr Turnbull supported his candidature for the UN job Mr Turnbull says the account of what happened is "absolutely untrue"

Mr Rudd released the letters on Friday night, after Mr Turnbull revealed he had decided not to nominate him for the role as the world's top-ranking diplomat.

In the three letters, dated earlier this year, Mr Rudd wrote that Mr Turnbull had previously told him he would support his bid.

"They were all confidential discussions, they were all private conversations," Mr Turnbull said on Monday.

"It says a lot about Mr Rudd that quite some time after the event he would seek to present an account of them in correspondence he would write to me with the clear intention of subsequently releasing it."

In one letter, dated May 1, Mr Rudd wrote to Mr Turnbull:

"You in fact sent me a message on your preferred Wickr system where you stated that you and the FM (Foreign Minister) were 'as one' in your support for my candidature."

"We continued this discussion further on Wednesday 23 December in your Sydney office. Once again you stated your support for my candidature.

"You added that when the time came to lodge my nomination, you now wanted to take it to Cabinet to avoid the perception of a 'captain's pick'.

"You also said to me that the Cabinet process would not change the outcome."

Account in letters is 'absolutely untrue', says PM

On Monday the Prime Minister refused to say whether he gave his personal support to Mr Rudd in his bid for the UN's top job — instead, he said he repeatedly told the former prime minister it would be up to Cabinet to decide.

Sorry, this video has expired Mr Turnbull said Kevin Rudd was "not well suited" to the UN role.

He said the letter's account of what happened on December 23 was "absolutely untrue ... it was made very clear to him that it was a matter for the Cabinet, it would be a difficult political decision and he was given no assurance that he would get the answer he sought".

Trade Minister Steven Ciobo denied the decision could set a precedent for governments who do not want to back former opponents for international roles.

"Some of the harshest criticism that we saw of Kevin Rudd and his abilities came from the Labor Party," he told Radio National.

"I certainly don't think that it's established a precedent in any way, shape of form."

Mr Rudd's chief backer in the Cabinet room was Foreign Minister Julie Bishop, who had argued he was a qualified candidate because he was a former prime minister.

Treasurer Scott Morrison told 2GB "she did her job, she put forward the recommendation which you'd expect the Foreign Minister to do."

Letter 1: April 4, 2016

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Letter 2: May 1, 2016

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Letter 3: July 28, 2016

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