Even then, it will be able to do so only by simple majority through a three-yearly confidence vote immediately after an election. To move against an incumbent leader at other times will require two-thirds of the caucus - a much higher threshold than the current simple majority to sign a petition.
The rules are similar to those of Ireland's Labour Party, and Party president Moira Coatsworth believes restricting the ways in which a leadership vote can be forced in such a way will provide more stability and give a leader a clear mandate between elections.
Labour is hoping the result of the changes will be a big sayonara to the "night of the long knives" scenarios, and to the damaging and distracting speculation about imminent leadership challenges that has bedevilled the party.
Coatsworth sees the provision of a two-thirds vote as a safety valve for "extraordinary circumstances" and grave concerns about a leader that must be dealt with.
However, the reality of politics is that if more than half of a caucus is unhappy with the leader, it can be more damaging to keep that leader on than to replace him or her.
In such a situation the split in a caucus and the resultant undermining of a leader's authority is highly corrosive and can damage the party.
Yet Labour's new rules make no provision for such a situation unless the leader can be pressured to stand down.
What the new rules do prevent is challenges from ambitious MPs at times when the leader is not widely unpopular. It will also blunt National's goading about the leadership.
National can and will still poke fun at the likes of David Cunliffe for their ambitions. But the new rules will take much of the sting out of those jokes.
The high threshold needed to force a challenge will make it very difficult for any challengers at times other than at the confidence vote immediately after an election - a time in which the leaders' fate has usually already been decided by their success or otherwise.
The new system stops short of the Canadian Labour Party system in which members themselves can force a leadership vote at party conferences.
But it is a major move for Labour and Shearer and Coatsworth have made much about this lurch to a more democratic process, without pointing out that "democracy" is both a blessing and a bane.
A range of possible nightmare scenarios emerge, not least of which is the prospect of somebody who is loathed by the caucus being elected courtesy of the members and unions. Recent history is instructive in this regard.
As Otago University's Bryce Edwards has pointed out, the new rules could have meant David Cunliffe, rather than David Shearer, becoming Labour's leader because the 60 per cent of the vote accorded to the members and unions could have overwhelmed the caucus portion.
During their election battle last December, the general theory was that many in the caucus were driven by the "ABC" theory - Anyone But Cunliffe. But Cunliffe was seen to have stronger support among the members, especially in Auckland. This was hypothetical, as there was no way to really gauge whether Cunliffe's support among the members was truly larger, or simply louder.
But it will now be possible for the members and unions to override the caucus' own preference even where it is unanimous.
Reserving the caucus the power to call a further leadership contest where two thirds of them agree could act as a form of retrospective veto in such a circumstance, although exercising that immediately would be messy. It would also be a very brave caucus that defied the membership in such a way without at least giving the leader a chance.
The changes will also make it harder to predict the outcome of a contest. It is easy enough to poll the small brigade of caucus members to see who has the edge, but nowhere near as easy to poll thousands of members and affiliate members.
This will be good news for the underdog candidate, because it puts an end to the sniffing of the wind process by which many caucus members tend to cast their votes. That involved waiting to see which way their colleagues are voting, and casting their own votes accordingly to ensure they were backing the right horse.
Those close to Cunliffe's campaign last year believed there was a strong element of this in the contest, and that Shearer's backers were spreading deliberately inflated numbers in their favour to scare the remaining undecideds into backing their man.
The split could also cause internal ructions and concerns have already been raised about the power the unions might have. The unions have 20 per cent of the vote - less than in the UK where MPs, members and unions have one third of the vote each.
That was critical for British Labour leader Ed Miliband, who won the title from his brother David courtesy of the union vote, although David had beaten him narrowly among MPs and party members.
But the power of those five affiliated unions could be further diluted by a further change decided on by the review which will allow other groups - such as Ratana Church and community organisations - to become formal affiliates.
In the meantime, Shearer has to get through his first caucus "endorsement" vote next February. Barring a disaster over the next six months there seems little doubt he will pass that. However, it must be tempting for Cunliffe to have another go, not least because the new processes are expected to be in place in December.
Those who fear the new system could lead to chaos could take comfort from the Green Party which has always had party members involved in selecting its co-leaders. The Green Party's leadership has tended to be long-term and stable.
However, there are significant differences: provided they stayed above the five per cent threshold the Greens have not been as susceptible to poll panic, nor do they have as large a caucus to contend with.
Last, but not least, the Green Party is electing a party leader. In Labour's case, as with National, the election is also of a potential Prime Minister and that ups the stakes considerably.
