An outgoing New Zealand First MP has slammed Winston Peters in an extraordinary interview - saying Peters' behaviour is increasingly erratic and he will form a coalition with Labour if at all possible.

Richard Prosser was demoted from third on the party list at the last election to 15th and will not return to Parliament.

Previously NZ First has been split and there is speculation it may happen again.

Political commentator Ben Thomas said National leader Bill English may be able to reel individual NZ First MPs over to his side.

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Someone like Shane Jones would be very comfortable in a National government, but that's unlikely to happen until at least the middle of the term, he said.

Meanwhile Prosser told the Herald that whoever negotiated with Peters in the coming days and weeks may need to deal with what Prosser called examples of erratic behaviour.

"He has always been Machiavellian, I guess, and a bit mercurial. And sort of says things in a way that leaves wriggle room. This goes beyond that. These are things when you just think, what on earth is happening?"

Prosser gave examples of recent behaviour including Peters interview with Radio NZ host Guyon Espiner, which he labelled "bizarre".

Another was Peters saying he had discussed with caucus the idea of withholding tax refunds from men who didn't have regular prostate health check-ups.

"I don't know where the hell that came from. He said the caucus had discussed it. Well, maybe that happened in a meeting that I missed but I don't remember missing a meeting."

He said the notion that Peters will consult with people in the party in order to make a decision about which party to support was "a myth".

"I think he will go left if he possibly can. And I think that's why he didn't close the door on the Greens. I think if he can work out an accommodation that uses their votes and possibly chucks them a couple of trinkets...and put Labour in, I think he will do that.

"The simplest thing and obviously what the country wants is for him to just sign up with National and just get on with the job of forming Government. But the nation has obviously indicated a mood for change, and if he can capitalise on that as well there is a lot of utu to go on with him and the Nats."

Asked what concessions Peters might push for, Prosser said it would "be more around baubles than policy".

"Because the policy goals that are most important to NZ First supporters are anathema to the Nats. And he won't get those - restrictions on foreign ownership...restrictions on immigration, he won't get any of those. There may be some minor bits and pieces."

In his speech to supporters last night Peters warned colleagues around the country to not say anything in the coming days that may embarrass the party.

Prosser, who said he was considering doing some work in broadcasting post-politics, said he didn't feel that comment was directed at him.

"I guess he is just wanting to reiterate that it is very important that nobody commits the cardinal sin of building themselves a personal profile, denying him some of the limelight that he clearly believes belongs to him alone.

"For the sake of the party going forward there has always been the need for succession...it gives the lie to the claim that New Zealand First is not Winston First.

"You look at the people that have raised their head above the parapet they have pretty much normally had them lopped off again."

During the election campaign Prosser gave a speech to a Business NZ conference in which he said his party wanted to bring back electricity assets to a simple state-owned, state-controlled umbrella.

Prosser said buy-backs would be done over time, but NZ First's stance was the institutions would be purchased back at the price they were sold for.

Peters subsequently put out a press statement saying Prosser's comment was a "throwaway line that was not fully explained".

"Had he had the time, he would have explained that the buyback of power companies would be at an appropriate time in the future," Peters said.

"That is, we would only be buying back shares when they became available."

Prosser sent the Herald emails to show NZ First had previously stated its intention to buy back shares at the price they were sold for. He said he found Peters' statement to be galling.

"It's fair to say I'm not amused. Certain machinations have gone on for some time behind the scenes.

"I think there is a space I think for a leader to have a little bit of humility, and fess up when they made a genuine mistake.

"Provided of course they are fully aware they have made a genuine mistake and I'm not certain that's the case."