UPDATE 5.05 More now from that Kevin Rudd press conference.

Mr Rudd says he didn't run for the leadership because he was told there was "zero" chance of winning a caucus ballot.

Prime Minister Julia Gillard called a spill of the Labor leadership on Thursday, but Mr Rudd did not challenge despite caucus members urging him to do so.

Mr Rudd told reporters in Brisbane this afternoon that had he challenged he would have been attacked for going back on a promise made in February 2012 never to challenge again and "split the Labor Party right down the middle".

He said he was advised in a meeting with supporters on Thursday that he did not have a majority of votes.

"I asked them `what are the prospects of us obtaining a significant majority' - their collective response was zero," he said.

"I asked them `what are the prospects of us obtaining a majority' - to which their response was zero."

Then, according to Mr Rudd, each told him "Kevin I believe you should not run".

Mr Rudd said was time for the Labor party to "unite totally" behind Ms Gillard.

"... Julia Gillard, has my 100 per cent support," he said.

"As I said in a written statement today, there are no circumstances under which I would return to the leadership of the Australian Labor Party in the future."

He would, he said, continue to offer his services as the federal member for Griffith.

Mr Rudd said it was time to confront a significant threat to Australia's future.

"That is Tony Abbott and all that he stands for.

A person I have said before I don't believe has the temperament or the polices to be the prime minister of the country."

He said the party must present to Australia its "phenomenal achievement" in keeping Australia out of the world recession that followed the global financial crisis.

UPDATE 4.20 Kevin Rudd has broken his silence and outlined the reasons why he did not contest the Labor leadership yesterday.

Mr Rudd says that, from the advice he received, he had 'zero' chance of gaining a majority.

He has also called for unity under Julia Gillard and says the Prime Minister has his '100 per cent' support.

He also says he did not expect Simon Crean's call for a leadership spill yesterday.

More to come.

UPDATE 3.40 And another one goes. Kim Carr is standing down as Human Services Minister.

"I want to make it perfectly clear that I have not made this decision lightly."

He insists the numbers were very, very close, had Kevin Rudd decided to stand yesterday.

He says the worst result for Labor would have been a narrow loss for Mr Rudd and it was in the best interests of the party that he did not run.

And we are hearing Mr Rudd will hold a press conference at 4.15pm (AEDST).

UPDATE 3.30 Minister for Human Services and Rudd supporter Kim Carr is also set to stand down. A press conference is scheduled for 3.40pm.

More form the Ferguson press conference as well.

"I have a view it is the only honourable thing to do," he told reporters.

Mr Ferguson supported Mr Rudd's challenge in February last year, when he said he was worried about the direction of the party.

"For those reasons I would have voted for Kevin Rudd yesterday and Simon Crean (as deputy) to try and give this party a fresh start," he said.

"Unfortunately the events of yesterday were not what we have hoped for.

"The caucus has made the decision. I respect that decision."

Mr Ferguson said by resigning he was giving the prime minister and the ALP the opportunity to regain the Labor legacy of reform and embrace an approach which looked after all Australians.

"In doing so, seek the best position for the party I have been a member for 35 years to win the next election," he said.

He also gave credit to Labor stalwart Mr Crean, who had planned to run for the deputy leader spot and called for the spill.

"I thought what he did yesterday was courageous. It was about trying to give the party a fresh start," he said.

"For this reason, I would have supported him."

UPDATE 2:47 Rudd supporter Martin Ferguson is standing down as Resources Minister.

He said in a press conference that he would have voted for Kevin Rudd had he stood yesterday.

He told reporters that he had been endorsed to stand at the next election and would be standing.

Ferguson spoke about the future of the Labor party, saying he believes there is now opportunity for a fresh start.

UPDATE 12.42 More Kevin Rudd backer tipped to go after the former PM announced he would never again lead the Labor party.

Mental Health minister Mark Butler and Human Services Minister Kim Carr - both strong Rudd backers - have reportedly been told they cannot be 'servant of two masters', News Ltd reports.

11.30 Kevin Rudd says there are now "no circumstances" under which he would return to the Labor leadership.

Mr Rudd says he had been consistent that he would not challenge for the leadership and that he would contest the next election as a local member of parliament.

"That position hasn't changed," a spokesman told AAP.

"Furthermore, Mr Rudd wishes to make 100 per cent clear to all members of the parliamentary Labor Party, including his own supporters, that there are no circumstances under which he will return to the Labor Party leadership in the future."

11am Prime Minister Julia Gillard has thanked Chris Bowen for his contributions to her government, after he resigned from her ministry.

The Prime Minister particularly thanks Mr Bowen for his work as Immigration Minister, while describing his decision to resign as 'honourable'.

10.20 Foreign Minister Bob Carr has addressed the media from Washington, saying Prime Minister Julia Gillard has his full support and he has no plans to resign.

The Prime Minister has emerged 'stronger, rather than weaker', from yesterday's leadership battle, Mr Carr said.

Mr Carr had been reported earlier this week as a Rudd backer, who was working behind the scenes to oust the Ms Gillard - a claim he has emphatically denied.

10.14am Senior minister and Kevin Rudd backer Chris Bowen has resigned from the Gillard government ministry.

Acknowledging his role in the failed leadership coup, Mr Bowen said he could no longer serve under the Prime Minister.

"I took the decision that the best thing for the Labor Party and for Australia would be for Kevin Rudd to return to the Prime Ministership''.

"Having expressed that view, having worked towards it... I decided to resign from the Gillard cabinet," he said.

"I think political watchers will know that for some period of time there's been an undercurrent in our party and it was dealt with yesterday and brought to an end."

"I made the call to support Kevin Rudd. I don't apologise for it."

Mr Bowen plans to re-contest the marginal western Sydney seat of McMahon at the September election.

The seat will be key to any chance the ALP has of retaining power.

9.30 The Prime Minister admits she was 'surprised' that Kevin Rudd didn't put his hand up in yesterday's spill of leadership positions.

The Prime Minister also told the ABC any future leadership speculation can not be taken seriously.

"I think if anybody wanders into a journalist's office in the future claiming to have the numbers sheet they will be met with laughter," Ms Gillard said.

"This is over."

LABOR IN CRISIS

Prime Minister Julia Gillard says she expects a number of her cabinet members will be considering their positions following the leadership spill.

Ms Gillard says she accepted the resignation of Richard Marles from his parliamentary secretary position on Thursday night.

"I anticipate there will be a few more people considering their position," she told ABC radio in Melbourne.

"They will do that. I will also consider the view as to what is best for the government over coming months, for the nation over coming months."

Ms Gillard said she had a ministerial reshuffle to deal with and would put together the "best team" to take the government to the September election.

Ms Gillard says the leadership issue is over.

"I think if anybody wanders into a journalists' office in the future from the Labor Party claiming to have a numbers sheet they will be met with gales of laughter. This is over," she said.

Ms Gillard said she did not think the media laws triggered the leadership spill.

"I think political watchers will know that for some period of time there's been an undercurrent in our party and it was dealt with yesterday and brought to an end," she said.

She dismissed Opposition Leader Tony Abbott's plan to put a motion of no confidence in her government at the next sitting of parliament.

"I've heard Mr Abbott huff and puff about no confidence motions over many occasions and it never comes to anything," she said.

"Something tells me he's thinking that he can get some media today saying that and then by the time we come back in May you won't hear anything of it."

Ms Gillard said she did not agree the business community was almost unanimously unhappy with her government.

"I meet with business people very routinely, even in the course of this week, which has had its stresses and strains, I've met with a number of business people," she said.

Ms Gillard said she had some big budget decisions to make.

The Mineral Resources Rent Tax (MRRT) had not delivered what was originally forecast and revenue was down across the board, which had "confounded" economic professionals.

She was committed to funding the NDIS and Gonski reforms despite the budget's difficult position.

"We do what we've done over the last five budgets, which is we identify savings, many of them very tough savings, and they cause moments of concern and moments of opposition," she said.

"... we make those savings in order to fund things that we think are a higher priority."