NZ First's MPs and board members are meeting to decide whether National or Labour will lead the next government, but it's not known when they'll reach a conclusion.
The meeting began about 11am and continued through lunchtime.
Party leader Winston Peters refused to comment to media when he arrived at Wellington airport earlier on Monday.
However about 6pm a NZ First spokesperson said there would be no public announcement tonight.
The board members have been booked in for an overnight stay. The meeting will be reconvened tomorrow.
One MP, Mark Patterson, arrived ahead of the meeting and said a lot of clarification had been needed over the weekend on the coalition negotiations held with the main parties last week.
He was not the only one signalling that it could take a little longer than Monday to make a decision.
National leader Bill English said his party was yet to talk to kingmaker Winston Peters about ministerial positions and the NZ First board would not have a completed agreement to sign off on.
"The discussion NZ First are having today is another step in the process, but it's by no means the final step in actually agreeing a government," English said.
NZ First 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.
A Greens' spokesman said about 170 delegates were "ready to go" for a teleconference that would sign off their support of any agreement. It would need 75 per cent support.
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.
It may alternatively choose to stay on the cross benches and abstaining on confidence votes, which would allow National to continue governing because it has the most seats.
NZ First has nine MPs and its board has 14 members, including Peters and his deputy Ron Mark.
Peters insists he won't make the decision and says it will be entirely up to his caucus and board.
Ardern and English are bound by a confidentiality agreement until Peters announces the decision.
A brief history of MMP governments since 1996
* 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.
* 1999. Labour, under Helen Clark, formed a coalition with the Alliance but the two parties didn't command a majority in parliament. 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.
* 2002. 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 Clark had the choice of three support partners - NZ First, the Greens and United Future. She negotiated a support agreement with United Future.
* 2005. National came back strongly but Labour again won more seats than its rival, 50 to National's 48. By now 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. Peters became Foreign Minister outside cabinet, and Peter Dunne Minister of Revenue, also outside cabinet. The Greens had no positions.
* 2008. 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 Maori Party. It was more than he needed and broadened his government. Ministerial positions were given to minor parties but they were not in cabinet.
* 2011. Key was by now a popular prime minister and brought National back with an increased share of the party vote - 47.3 per cent which gave the party 59 seats. It was a simple matter for Key to again sign up the minor parties in support roles and the minority government was able to maintain its stability.
* 2014. 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 Key was again running a minority government with support from his old allies ACT, United Future and the Maori Party. In November 2016 Key resigned and left parliament. Bill English was chosen by National's caucus to take over as prime minister.
* 2017. 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. Winston Peters was again the kingmaker.