An announcement by NZ First leader Winston Peters about which of the main parties will lead the next government has been put off until at least Tuesday.
The party's caucus and board began a meeting around 11am on Monday which went on into the night, and a spokeswoman said it would continue on Tuesday morning.
Mr Peters hasn't commented on the discussions.
One MP who did, Mark Patterson, said a lot of clarification had been needed over the weekend on the coalition negotiations held with the main parties last week.
There seems to be more ends to be tied up than NZ First had believed when Mr Peters said on Friday the negotiations were over.
National leader Bill English has also indicated it could take longer than anticipated to reach a conclusion.
He said early on Monday his party was yet to talk to Mr Peters about ministerial positions or government arrangements.
"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," Mr 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 Mr Peters and his deputy Ron Mark.
Mr Peters insists he won't make the decision and says it will be entirely up to his caucus and board.
Ms Ardern and Mr English are bound by a confidentiality agreement until Mr Peters announces the decision.