Bill English has praised Winston Peters' "tough" approach to negotiations, saying the Greens could learn a thing or two from the veteran MP.
NZ First began its coalition talks with both National and Labour on Sunday, with leader Mr Peters suggesting a decision will be made by Thursday on just who will form the next Government.
The discussions come more than two weeks after election, which saw voters split almost right down the middle - giving National 56 seats and the Labour-Greens bloc 54, with ACT getting one and NZ First, nine. At least 61 are needed for a majority in Parliament.
"Mr Peters, as you would expect, is using the weight of the position he has to make gains," the National Party leader told The AM Show on Monday.
Some high-profile National supporters have been pushing for a National-Green deal, avoiding the need to deal with Mr Peters altogether. The Greens said it was unlikely a deal could be done, but would listen if Mr English gave them a call. He didn't.
"The Green Party don't appear to understand the position they're in or could have been in," said Mr English.
"[Mr Peters is] playing his hand with a great deal more assertion than the Greens... He's a tough negotiator and he understands the position he's in."
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Mr Peters should keep in mind his party got nine seats while National got 56, Mr English said, adding that a repeat of the drawn-out negotiations of 1996 wasn't likely.
"NZ First's vote is quite a lot smaller in 2017 than it was in 1996, and ours is quite a bit higher. That alters the dynamics a bit."
Mr English disagrees with pundits who say as the largest party, National has a right to form the Government.
"It's not a right - that's why we're having a negotiation. If it was a right, there wouldn't be all this process going on."
If Labour, the Greens and NZ First manage to cobble together a coalition, Mr English believes it would be a legitimate Government.
"A Government is a Government if it gets a majority in Parliament."
But as talks with Mr Peters so far have been "constructive and civilised", Mr English said he's still confident of keeping his job as Prime Minister.