New Zealand First leader Winston Peters says he will be consulting with colleagues on post-election plans.
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National and ACT together have 61 seats in a 121-seat Parliament, while Labour support fell to just under 27 percent.
New Zealand First gained about 6.5 percent of the vote which could give it eight MPs.
National leader Christopher Luxon said this morning he hadn't spoken to Peters, and repeated his comment of last night that he appreciated Peters' comments offering to help if needed.
Addressing supporters last night Peters suggested there were a lot of National's promises that were not affordable and he would work to rein them in, given the chance. "If we can help, going forward, we will," he said.
Peters this morning gave no indication of what his position would be in any discussions.
"My job is to consult with my colleagues - we haven't had a chance to talk to each other.
"When we've decided what we're going to do, and what we're doing it with, we'll all let you know."
He said the New Zealand people would soon start realising the country was in an "economic and social crisis" and once they did some of the promises in the campaign "won't be worth confetti".
Peters said this it was the toughest campaign he had fought "by a long shot".
"We were given no chance and it showed in the lack of coverage and demonising of New Zealand First," he said.
"We were ... marginalised, shut out by the mainstream media the whole way."
Asked whether he would consider being part of government if there was a chance to do so, he said: "Do the maths and wait".
Deputy leader Shane Jones earlier told Morning Report "no-one should write off the influence of that New Zealand First can have" once all the votes were counted.
It was possible National could lose ground in the special vote count, he said.
"If that is the case then obviously that changes the power calculus but at this stage we're backing the fact that the movement's back in Parliament and we'll wait and see where the dust finally settles."
It was too early to tell if the party would want to be part of government again, he told Morning Report.
Jones said the party was seeing the dividends of two years of hard work to get back into Parliament.
Former New Zealand First minister Ron Mark told the programme the party was in a good position to "lean in" to a new government if required.
He said Peters was likely to want to take stock and wait for special votes to be counted before making any decisions.