The left faction has an internal vote to choose its preferred candidates to go forward to the preselection proper. In Murrumbidgee, the Woden-Weston electorate, it voted Rebecca Cody, who has stood before, as the No 1 candidate, a prominent indigenous leader who asked late on Wednesday for her name to be withheld as No 2, and Mr Corbell as No 3. One source said they received 69, 61 and 36 votes respectively.

The result was such a blow to Mr Corbell that he withdrew his candidacy and announced he would resign from the Assembly at the election, leaving Labor without a sitting candidate in the electorate, which is based around Woden and Weston.

The Liberals, in contrast, have high-profile candidates leading their team in the 2016 election – Leader Jeremy Hanson and Giulia Jones.

At least eight Labor newcomers are now vying for the five spots on Labor's ticket in Murrumbidgee, with two likely to win spots in the Assembly. As well as the two left wingers, there are three from the right and two independents, among them Mark Kulasingham and Brianna Heseltine. Ms Heseltine has not confirmed.

Mr Corbell said he would not talk about internal party matters. But he insisted that if he had kept his name in the pool he would have won a spot.

"If I had put my name on the ballot I was confident I would have been preselected," he said " ... It's a rank and file ballot of all party members."

As it happened, once he was put in third spot he wasted no time in announcing his resignation, but Mr Corbell rejected the suggestion his decision was made impulsively.

"The simple position is that I came to the conclusion that after what would be 20 years that it was time to do other things. It was not a decision I came to quickly. It was certainly not a decision I came to in an impetuous ... way. I took that decision after a very long period of reflection."

It is unclear why Mr Corbell was put last on the ticket. One interpretation floated this week was that it was an unexpected result, with voters in the Left assuming Mr Corbell's election and focusing on the race for second and third spot. Other Labor figures rejected the randomness implied by that explanation, insisting factional votes are to a large degree orchestrated by party heavyweights.

Most refused to speak on the record. Asked why the party would seek to deliver such a blunt message to the only sitting politician in the seat, party figures either denied the premise or suggested it was to do with Mr Corbell's refusal to engage enthusiastically with internal party processes and campaigning.

One of the powerful figures in the Left is Dean Hall, of the Construction, Forestry, Mining and Energy Union in Canberra. Asked about his role, Mr Hall said he was "a big fan of Simon" who would have been his No 1 choice in the electorate if he had stood.

"I reckon he's a great operator, I think he's done fantastic things. he's got a huge intellect and he's been a big friend of construction workers," Mr Hall said. "We'll miss his intellect and drive. But on the other hand I can understand, 19 years in public life as a politician would have to test anybody."

Mr Hall said it had been taken as a given in the left preselection that Mr Corbell had the support, with the vote more about a fight for second spot.

Mr Corbell's departure will leave Yvette Berry and Mick Gentleman as the senior Left parliamentarians. Ms Berry's father, Wayne Berry, was a factional powerbroker in his time but when approached on Wednesday said his longstanding position was never to comment on internal party affairs.

Mr Gentleman is overseas and Ms Berry declined to comment.