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He said he had little doubt Mr Dotcom was simply trying to get politicians in place who might be able to help block his extradition.
"You've got a guy who can't buy a house in New Zealand, but he can buy a political party. I think most New Zealanders would look at that and be pretty cynical about it. No one should be under any illusion.
"This is a very wealthy guy trying to buy a political party to stop himself being extradited."
He said Ms Harre and Mr Harawira had "zero" in common with Dotcom.
"Kim Dotcom lives in a house that is probably worth about $25 million. FBI records show the hundreds of millions of dollars that came with him.
"And you're telling me he's a natural bedfellow of Laila Harre and Hone Harawira? If you believe that then you believe Santa Claus is going to turn up 12 times a year just to make New Zealand a more joyous place."
The Internet Mana deal is aimed at maximising the number of MPs Harawira can bring into Parliament with him if he holds his Te Tai Tokerau electorate.
He said he had not known the law allowed parties to stand as joint force and then split into separate parties after the election and "it seems a bit of a rort."
Mr Key said there was a difference between that and the deals National struck over electorates with Act in Epsom and United Future's Peter Dunne in Ohariu. "Those people win their seats outright, in their own right. Their motivations are the beliefs of those parties. That's not the case here."
He said the Internet Mana Party's first joint policy of free tertiary education was "unaffordable and New Zealanders will know that."
"That's what you're going to see from the far left of politics. You'll be led by Russel Norman, Kim Dotcom, Mana, David Cunliffe, you're going to see extreme examples that New Zealand can't afford and fundamentally promises they can never meet."
Mr Key said he was not worried about the impact of the Internet Mana Party on National because it would take votes from the Greens and "disaffected Labour voters" rather than National.
"From National's point of view it's of no great relevance. But if you're Metiria Turei, Russel Norman, maybe David Cunliffe, you might be just a little more worried."
- By Claire Trevett of the NZ Herald