David Cunliffe's leadership will be put to an early test at the Labour Party conference this weekend as he mediates between two divergent forces in the party on the Trans-Pacific Partnership.
Labour's bipartisan approach to free trade could be over if a strong group of TPP sceptics within the party win the day, and the Government is privately worried.
The party's rows over leadership are now in the past - the battle royal has now turned to policy.
Labour's affiliated unions have a role in the latest battle and are deeply suspicious of the TPP, as are a group in the caucus including former deputy leader Grant Robertson and Te Atatu MP Phil Twyford.
The other group, championed by former Trade Minister Phil Goff, is willing to put greater faith in the upsides of TPP.
Labour's TPP sceptics were bolstered by the passage of a remit at the Council of Trade Unions conference last month which simply stated: "That this conference opposes the Tran-Pacific Partnership Agreement."
It was passed unanimously, with no speeches in opposition.
One of the main fears of the sceptics is that the deal could limit the powers of Government in the future to regulate investment, to favour New Zealand businesses, and could constrain it from coming up with policies such as the creation of NZ Power - a single buyer of wholesale electricity.
The sceptics see it essentially as a brake on future interventionist Government, which is much more Labour's style than National's.
Mr Goff has been critical of the Government for not doing more to explain the broad negotiating positions of the TPP to the public and for sometimes simply dismissing critics rather than addressing their concerns.
He will be its chief advocate at the party conference, but is not confident he will win the debate.
"I really don't know, because delegates will have been exposed to all of the concerns about TPP but not about the advantages of it. That creates a challenge."
The most intense debate on TPP is likely to occur when delegates talk about the Policy Platform - a new document of principles and values with which specific policy must be compatible.
The proposed wording on trade deals says: "We will only support trade agreements that protect New Zealand's sovereign right to make law and regulations as we see fit."
One of the questions is whether that means the party could not support a deal that reduced in any way New Zealand's sovereign right to make law or whether it leaves wiggle room. It has been suggested that there may be amendments put in to toughen it up.
Mr Goff acknowledges the well-accepted precept that trade deals - along with every other international agreement - always reduce a country's sovereignty.
"Not just trade negotiations but any international agreement we sign up to, including the International Convention on the Protection of Civil and Political Rights that removes our sovereign right to persecute people [and] the Convention Against Torture, which removes our sovereign right to torture people. There are a whole lot of things that involve the surrender of our sovereignty that every delegate at the party conference would actually approve." He said many people were concerned about the Australian case in which the Philip Morris tobacco company challenged - under the investor-state dispute clause of a free-trade agreement - the Government's right to introduce plain packaging for cigarettes.
Mr Goff said he had been briefed by New Zealand's chief TPP negotiator, David Walker, with the approval of Trade Minister Tim Groser, and he felt confident that New Zealand was fighting hard on core issues that, had there been a Labour Government, would have been the same.
"My understanding of what New Zealand's position will be on this is that we would absolutely die in a ditch to protect our right to regulate or legislate in the public good and that is a bottom line," Mr Goff said.
He also said the Australian case was the result of poor wording in a trade deal that NZ would never let through.
Mr Cunliffe told the Herald that Labour was traditionally an internationalist party with a strong record of concluding good-quality trade agreements.
While the TPP clearly had an upside, it also had domestic fishhooks - particularly relating to Pharmac, intellectual property and investor-state disputes procedures. The debate required more information in the public domain to assess the pros and cons.
"So Labour is reserving its position until that information is in the public domain."