Winston Peters' NZ First party is about to start a meeting that will decide whether National or Labour leads the next Government.
It holds the balance of power and can either grant National a fourth term or make Jacinda Ardern Prime Minister in a Labour-led government.
If it goes to the centre-left, the Greens have to be part of the deal because NZ First and Labour don't have enough seats between them for a majority.
NZ First could decide to form a full coalition Government with its MPs holding Cabinet positions, or go for a looser support agreement with ministerial positions outside Cabinet.
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It also has the option of staying on the crossbenches and abstaining on confidence votes, which would allow National to continue governing because it has the most seats. That's unlikely given last week's intense policy negotiations with both the main parties.
NZ First has nine MPs and its board has 14 members, including Mr Peters and his deputy Ron Mark.
Throughout the secret negotiations there's been not even a hint of which way NZ First is leaning.
Mr Peters insists he won't make the decision and says it will be entirely up to his caucus and board.
They'll be weighing up documents which set out the policy concessions National and Labour offered during the negotiations. The concessions determine how many of NZ First's policies will be implemented by the next Government, and to what extent.
Ms Ardern and Prime Minister Bill English are bound by a confidentiality agreement until Mr Peters announces the decision, which could be later on Monday or on Tuesday.
A brief history of MMP Governments since 1996
The first MMP government was formed by National and NZ First. It took six weeks to negotiate and was a full coalition, with Cabinet ministers from both parties. Winston Peters was Treasurer, a contrived position, and deputy Prime Minister. It fell apart after Jim Bolger was ousted as Prime Minister and replaced by Jenny Shipley, who lost the next election.
Labour, under Helen Clark, formed a coalition with the Alliance but the two parties didn't command a majority in parliament. Ms Clark needed the Greens, who had won seven seats. They agreed to a support arrangement which gave the Government sufficient votes on confidence and supply to hold office. The Greens were not part of the Government.
National was trounced and Labour went into coalition with the Progressives, a splinter of the Alliance, which held two seats. But they were still short of a majority and Ms Clark had the choice of three support partners - NZ First, the Greens and United Future. She negotiated a support agreement with United Future.
National came back strongly but Labour again won more seats than its rival, 50 to National's 48. By now Ms Clark was accustomed to running a minority Government with the support of the smaller parties, and she negotiated agreements with NZ First, United Future and the Greens. Mr Peters became Foreign Minister outside Cabinet, and Peter Dunne Minister of Revenue, also outside Cabinet. The Greens had no positions.
National, now under John Key, won 58 seats against Labour's 43. Key decided to run a minority government with support agreements from ACT, United Future and later the Māori Party. It was more than he needed and broadened his government. Ministerial positions were given to minor parties but they were not in cabinet.
Mr Key was by now a popular Prime Minister and brought National back with an increased share of the party vote - 47.3 percent which gave the party 59 seats. It was a simple matter for Mr Key to again sign up the minor parties in support roles and the minority government was able to maintain its stability.
Labour's turn to be trounced and on election night National held 61 seats, the first outright majority by a single party since MMP was introduced. But it lost a seat on special votes and Mr Key was again running a minority government with support from his old allies ACT, United Future and the Māori Party. In November 2016 Mr Key resigned and left parliament. Bill English was chosen by National's caucus to take over as Prime Minister.
National, under English, won the most seats - 56 to Labour's 46. But Labour, with the Greens and NZ First, had enough seats for a majority. National could also hold a majority and stay in power, but not without NZ First. Mr Peters was again the kingmaker.