A group of Labour MPs were planning a motion of no confidence against party leader David Shearer at the caucus meeting on Tuesday - a step pre-empted by his resignation from the leadership yesterday.
Mr Shearer said he was stepping down because he had not achieved the desired results.
After taking "soundings" from some of his colleagues, he believed he had lost the confidence of many Labour MPs, and it was time for a change before next year's election.
He said there was no challenge against him.
But the Herald has learned MP Maryan Street was preparing a motion of no confidence in Mr Shearer for Tuesday's meeting.
Plans were also being made to send a delegation to him before that to ask him to stand down rather than force the confidence vote.
The MPs involved were certain the motion would have succeeded if it had been required.
Ms Street would not comment yesterday, but it is understood she decided to front the motion because of growing concerns among MPs over Mr Shearer's inability to fire as leader and his poor poll ratings.
A source said there had been discussions for months, but nobody was willing to force the issue until Ms Street stepped up.
One Labour source also said former Prime Minister Helen Clark had been "active" on the issue while in New Zealand over the past fortnight, speaking to some MPs about it.
It is not known if Mr Shearer knew the no-confidence motion was coming, although some MPs close to him had heard rumours of it.
Many of Mr Shearer's front-bench colleagues had confronted him several weeks ago to tell him that he had to lift the party's performance and that some MPs were getting restless.
Mr Shearer said he would stay on as an MP and whoever replaced him would have his full support as leader.
"My sense is I no longer have the full confidence of many of my caucus colleagues. From the soundings I have taken I believe it is better that I step down so we can have a clean change to a new leader who can take Labour through to victory in 2014."
The resignation will trigger Labour's new leadership selection rules for the first time, giving votes to party members and affiliated unions, as well as MPs.
The most likely contenders are David Cunliffe and Grant Robertson; both have said they will decide over the next few days whether to run.
Mr Cunliffe said Mr Shearer's decision had come as a surprise and he did not know what had prompted him to believe he had lost the confidence of his caucus.
"We respect he has come to a decision he believes is in the best interests of the Labour Party."
Mr Robertson said he had been a loyal deputy to Mr Shearer, and had supported him throughout his 20 months as leader.
"Everybody in the caucus, as everybody in the wider Labour Party, would like to see us doing better in the polls and David has obviously reflected on that. He made his own decision."
Andrew Little would not rule out seeking the leadership, but because he is a first-term MP he may be wary of making the run before he is ready.
Shane Jones is also unlikely to contest it because he believes he would not get the support. David Parker and Phil Goff both ruled it out.
Although Mr Cunliffe lost to Mr Shearer last year, the rule changes could work in his favour because he is considered to have strong support among party members, especially in Auckland.
He may struggle to get the caucus vote, but one anti-Cunliffe MP said yesterday the sentiment against him was not as strong as it had been, and many MPs would be happy with either of the two men as leaders.
Although Mr Shearer will stay on as leader until his replacement is elected, it is understood he is now on leave for the next few weeks - about the same amount of time the leadership contest will take.
Labour MPs yesterday said they were saddened by Mr Shearer's decision, and described him as a man of integrity who had done his best. But none said he had made the wrong decision.