Caretaker Prime Minister Bill English and Labour's Jacinda Ardern will know in just a few hours which of them will lead the next Government.
Winston Peters will announce his party's decision on Thursday afternoon. He will say which of the main parties NZ First has chosen, and what form the Government will take.
It could be a full coalition Government with NZ First ministers inside Cabinet, or a looser support agreement with ministers outside Cabinet.
Mr Peters released a media statement on Wednesday night saying he would be "in a position tomorrow afternoon to make an announcement on the result of negotiations following the 2017 general election".
He said he had spoken to Mr English and Ms Ardern and advised them of that "amongst other matters".
There were no indications on Wednesday night that he had told them his decision.
The "others matters" were thought to involve ministerial positions for NZ First MPs and the form the Government will take.
National's caucus will meet on Thursday morning for "an update on coalition talks", a spokeswoman for Mr English said on Wednesday night. There will be a teleconference with National's board after that.
The caucus and the board are expected to sign off an agreement without knowing whether it will be implemented.
"We stress that we have had no indication of what decision New Zealand First will make," the spokeswoman said.
Labour is expected to go through a similar process with its caucus and council.
The main party leaders have been talking to their senior MPs since NZ First began discussing its options on Monday last week.
National and Labour know NZ First's terms for an agreement and what they're giving up to get it.
It's not known whether Mr Peters will tell the leaders of his decision before he makes the announcement.