Doing an SDP analogue in NZ is quite difficult due to the fact that our Labour Party was already the SDP, so this is the best you're going to get.
List of Leaders of the Democratic Labour Party
1981-1982: Gang of Four (Gerald O’Brien, John Kirk, Brian MacDonnell, Mel Courtney)
1982-1984: Gerald O’Brien
1984-1987: John Kirk
1987-1988: Richard Prebble
In 1981, Bill Rowling lost his third consecutive general election as leader of the Labour Party. Under his visionless leadership, Labour had become home to liberals who knew nothing of economic policy, exercised more by anti-nuclear and feminist stances than by bread-and-butter issues. This gave rise to tensions that would erupt shortly afterwards into open fission.
The old guard of Catholics and workerists were deeply unhappy with the prominence of MPs and activists who wanted nothing less than the legalisation of things like abortion and homosexuality. They didn’t have much of an ideological quarrel with Rowling, who was very moderate, but they worried that the balance of power within the Party was moving in the direction of the extremists. Rowling did nothing to stem the tide, but he also refused to resign as Leader and bring the issue to a head. In frustration, group of relatively prominent rebels issued the ‘Grey Lynn Declaration’ and announced the formation of a new political party.
The main figure in the Gang of Four was Gerald O’Brien, a former MP and associate of Norman Kirk who had a sympathetic relationship with the existing third party, Social Credit, and was also the subject of whispered rumours of homosexual activity. However, there were others: John Kirk was a sitting MP for a Defence Force seat and had not yet divested himself of his good looks; Brian MacDonnell was a former Minister; and Mel Courtney also existed. These four were joined by several other sitting Labour MPs, including Geoff Braybrooke, Basil Arthur and Gerald Wall. There was even a National defector, Dail Jones, although nobody had ever heard of him.
The Democratic Labour Party (as it was called, in reference to the Australian party of the same name which also served socially conservative Catholics who traditionally supported Labour) leaped ahead in the polls and effectively ended Rowling’s leadership – however, he was replaced by David Lange, exactly the sort of person the DLP defectors hadn’t wanted to serve under. The new party formed a national organisation and elected Gerald O’Brien as leader over John Kirk – the vote was mostly fought on the issue of the DLP’s relation to Social Credit.
Unbeknownst to the membership at large, Bruce Beetham of Social Credit (also fighting an internal battle on abortion) had invited O’Brien to join the Socreds and lead them into the next election, but O’Brien aimed to carry more of the socially conservative Labour vote over with him in order to change the balance in Social Credit and perhaps also to break the mould of NZ politics. Hence his participation in the Gang of Four. John Kirk, meanwhile, distrusted the Social Credit Party, with its canny electioneering tactics and its confused political positions, and wanted the DLP to maintain a separate identity in any Alliance that could be formed. The victory of O’Brien, therefore, set the tone for the next few years of DLP-Socred relations.
As predicted, the two third parties formed an Alliance and fought the 1984 election together. Lange’s Labour failed to set the pace and Lange himself was written off as a joke, but he remained ahead in both the popular vote and (by far) in the seat counts, maintaining Labour as the Official Opposition. However, with the aid of a stalwart performance in a foreign policy crisis sparked by France reopening nuclear testing sites in the South Pacific, Prime Minister Muldoon rode a patriotic wave to an overall majority. But the day after election night, it was revealed that the NZ Dollar was in a critical state and needed to be devalued. Muldoon was pushed out quickly by Derek Quigley, a long-term free-marketeer critic of his economic policies, who stabilised the situation.
Quigley went on to drastically restructure the New Zealand economy, cutting taxes, privatising state assets and selling off state homes – a necessary bribe with which to tempt an electorate suffering from grievous unemployment. The forestry workers’ strike raged on for a year, but the public became increasingly hacked off with the militant trade union leaders and the timber mills were eventually closed. Entire towns lost their sole employer overnight. It was a harsh time, only made tolerable by a stock market boom which itself turned sour in 1987. And opposition to Quigleyism was of a poor standard: Labour messed around with talk of legalising homosexuality, while the Alliance response to the new situation was deeply confused.
Beetham was happy to condemn the changes and call for a return to the 1970s, leavened with a bit more fairness and an expansionary monetary policy, and so was Gerald O’Brien. But O’Brien was no longer DLP leader, having resigned as a scapegoat for their poor electoral performance within the FPP system. John Kirk replaced him, and was keener to stake out a separate platform for the DLP. He became a critical friend of the Quigleyites, praising their aspirational approach to state house sell-offs, yet also calling for Distributism and greater worker participation on the German Social Market model – which itself was inspired by Catholic social teaching. Left-wing critics said they didn’t see the value in involving the workers in decisions if the only topic on the agenda was which of them would be made redundant that week.
Kirk’s attempts to save the DLP from submersion into Social Credit were too little, too late. Barring a victory in the Timaru by-election of 1985, there was nothing to keep the momentum going, and the O’Brienites in the party moved to put a merger forward. Kirk’s faction failed to prevent inter-party talks from being set in motion by Conference, and he therefore resigned as leader, replaced by a younger MP, Richard Prebble. Kirk hoped that Prebble, hitherto aligned with the Kirkites, would hold the Socreds to a good deal, but the merger and the combined manifesto were widely regarded as dire embarrassments.
The parties combined in 1988, initially as the Labour and Credit Democrats (or, colloquially, ‘Le CiD’), but when that proved too unwieldy, they simplified it to The Democrats. Beetham and Prebble led it together at first, but Beetham was long past his best and Prebble was useless, so they were replaced very quickly by Terry Heffernan. Meanwhile, John Kirk and a few of his allies started a Continuity Democratic Labour Party, but they all lost their seats in 1990 and the party sank without a trace.
The Democrats still exist today, and have even participated in a coalition government, but the true legacy of the Democratic Labour Party was surely to be felt in the Labour Party itself: in 1996, Labour finally returned to office after two decades under the socially conservative and economically neoliberal Phil Goff. He governed essentially as the DLP would have done, and Labour has only dared return to the liberal, environmentalist and intersectionalist ideas of the early 80s in recent years, with the election of their ludicrous new leader, Helen Clark.
(As a side note, the Liberals in the UK gradually died after a very successful time in the 70s, and the main third parties in the 90s were an Alliance consisting of surviving Liberals, lefties and Greens on the one hand, and the populist Anti-Federalists on the other. Also they use MMP now, fuck you.)