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Prosser was part of a political panel at the Business NZ conference at Te Papa, and said his party would bring electricity assets back to a simple state-owned, state-controlled umbrella.
"That means if you have shares in Contact [Energy] - get rid of them now."
A representative from Mercury Energy asked Prosser after the session how he proposed to fund such a buy-back of electricity companies, given it would cost at least $11 billion.
Prosser said that would be done over time, but NZ First's stance was the institutions would be purchased back at the price they were sold for.
Seymour addressed that statement early in his own speech to the conference, saying it was an example of how reliable economic management could crash down in four weeks' time.
"The idea that you would have somebody who pretends to hold the balance of power - to come and tell you that a stock trading at $5.85 is going to be nationalised at $3.10 and you better all sell it.
"Well, I realise in a role such as mine you are supposed to have a certain amount of decorum. But that makes me really angry - what a f***ing idiot.
"That is someone who can barely manage his own diet and exercise regime," Seymour said, to groans from the audience.
"And he thinks he is going to be managing a ministerial post. And actually could be - we shouldn't joke about it."
Seymour said because Winston Peters might not be back in 2020, the restraints on NZ First MPs would loosen.
"Make no mistake - people who want to see stable, orthodox economic policy have one hell of a lot to worry about. And believe you me, I'm one of you."
Seymour has a history of controversial one-liners, including when he spoke about national symbols during the flag change debate, saying, "the French, for instance, love the coq".
Prosser is a long-serving NZ First MP, who made headlines in 2013 after he wrote that young Muslim men should be barred from flying on Western airlines, saying New Zealanders' rights were being denigrated by a pack of misogynists from Wogistan. He later apologised.
The Act leader has frequently clashed with NZ First leader Winston Peters, including on Twitter recently when Peters likened him to a chihuahua.