Prime Minister John Key says Winston Peters "could be right" when he says he'll decide the next Government.
But he's warning the veteran New Zealand First leader shouldn't be writing off the Greens just yet.
At his party's annual conference at the weekend, Mr Peters said he expects NZ First to "do better than we've ever done before by miles".
To do that, NZ First will need to win more than 13 percent of the party vote, which it last managed in 1996. Last year, it got 8.6 percent.
The latest 3 News-Reid Research poll has New Zealand First on 8.4 percent, enough to win it 10 MPs and hold the balance of power.
"Under MMP of course, he could be right – because you just never really know," says Mr Key.
"Even if you're leading by a big margin – we are in the 3 News poll for instance, and TV1's poll or whatever – you know, MMP's a fickle beast and it almost always demands that you do deals. But the thing is, it's so far out. We're over two years to go."
The Greens currently have more MPs than NZ First, and have done so since 2008. Mr Peters says they'll never be in power, and their failure to grow they party vote in 2014 cost the left the election.
But Mr Key says they could be a force to be reckoned with under the new co-leadership of James Shaw, who replaced Russel Norman earlier this year.
"James Shaw looks a bit more commercial shall we say, for want of a better term… than Metiria Turei."
And whether Mr Shaw's background in business means they could cut a deal with National remains to be seen.
"You never really know, and it depends on the rest of them, whether they'd be interested," says Mr Key.
He isn't ruling out doing a deal with NZ First at the next election, like he did in 2008 and 2011, and is confident National's losing of the Northland electorate to Mr Peters in the byelection in March can be undone.
"Oh well. It's reversible, isn't it? Like all vasectomies really, if they're well-done, I'm sure."
Mr Key infamously revealed at a news conference in 2010 he'd had a vasectomy.