In changes to Labour's rules at its conference today, the delegates made a surprise decision to allow 40 per cent of caucus to force a vote on the leadership in what is to be a regular confidence vote in the three months after an election. At other times, a majority vote is needed.
Mr Shearer will face his confidence vote on February 13 next year and the change means that only 14 of Labour's 34 caucus members can force a full vote on the leadership.
That would mean Labour's members and union affiliates voted - which could be enough to give the leadership to Mr Cunliffe.
Yesterday after the amendment was passed, Mr Cunliffe refused to rule out a challenge and would not say if he would support Mr Shearer next February.
Mr Cunliffe - who supported the amendment - said the leader did have his support but would not commit to endorsing him next February.
"I have not made any comment or decisions about what might happen next February because we don't know what's going to happen next year. We don't know what the future holds."
Mr Shearer said he was confident he could get the 60 per cent of caucus support to retain the leadership. He would not rule out disciplining Mr Cunliffe for any disloyalty, but said Mr Cunliffe had committed to him last week.
"I can only take him at his word, on what he said last week."
Asked if he believed Mr Cunliffe was being disloyal, he repeated his claim that he would be leader in 2014.
He would not discuss whether he would discipline Mr Cunliffe if he was openly disloyal.
Mr Shearer said he did not believe his position was more precarious and was confident he would pass the confidence vote in February.
He said he did not know what the gap between himself and Mr Cunliffe was in last December's leadership contest because it was a closed ballot, but was confident caucus backed him.
"I am confident I will have the numbers next year to ensure I am the leader going into 2014 - I can guarantee that."
He said overall the changes made the party more democratic and transparent by giving members a greater say in how the party was run.
"It is one of those things the membership decides. When we started this process that was the whole point of the opening up of the party and you take what the membership decides and you run with it - that's the way it goes."
He said he did not intend to dwell on the leadership issue, preferring instead to "focus on what New Zealanders are really concerned about, which is jobs and the economy".
Mr Shearer did not vote on the amendment, citing conflict of interest.
Mr Cunliffe said he had voted in accordance with his local electorate committee's wishes but he did personally support it.
"I think it's logical as well. This is simply the threshold which allows the three tranches - the membership, the unions and caucus - to all participate should the need arise in future."
Other Labour MPs had opposed the amendment, including Maryan Street, Chris Hipkins, and Andrew Little while Lianne Dalziel supported it. Ms Street said it would effectively allow "a tyranny of 40 per cent" to overthrow a leader who had been elected by a majority of the party.