Bill English or Jacinda Ardern may not find out NZ First's decision on which of them will lead government until the rest of the country does.

Kingmaker Winston Peters says no decision has been made on whether the National or Labour leader will be told in advance of his announcement, expected on Thursday afternoon.

"We haven't considered those sorts of things but obviously those are sorts of things we have to consider as well," he told Newstalk ZB, adding that he expects to be the one making the announcement.

A final decision is yet to be made. The party's caucus and board will hold final discussions during the day on Thursday before the announcement some time in the afternoon.

Mr English, who has been caretaker prime minister since the election, will lead the National Party into a caucus meeting on Thursday morning.

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," a spokeswoman said.

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Mr English said the same arriving at parliament on Thursday morning, while some of his other colleagues looked and sounded optimistic.

Ms Ardern has not been spotted at parliament and has not revealed her party's plans ahead of the announcement but 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.