Labour leader David Shearer says he always found it difficult to understand how Prime Minister John Key did not know about internet millionaire Kim Dotcom.

Speaking on TVNZ's Q+A, Dotcom said yesterday that Key was not telling the truth about what he knew before the German internet giant's arrest on copyright charges.

But he declined to produce evidence, saying that would be revealed in court.

Key said yesterday that he stood by his earlier statements that the first time he heard about Dotcom was on January 19, when he was briefed about a raid on the businessman's Coatesville mansion.

Shearer said either someone did not tell Key what was going on or he was "conveniently forgetting".

"It comes to the real issue about competence in Government and knowing what's going on."

Dotcom said it was a "monster issue".

"He is running around naked and telling people he has clothes on. Everyone can see he's naked. It doesn't make any sense. I know for a fact John Key knew before January 19. It's going to be a topic in our court hearings. I don't want to reveal too much.

"But I know. And for him not to be honest and upfront about it - to me there is really only one reason. That's the political-interference element that could stop the full extradition procedure."

Asked by interviewer Paul Holmes if he believed Key lied, Dotcom replied: "Yes". Asked if he had the evidence to prove that, he also replied: "Yes".

Key has repeatedly said he had never heard of the German millionaire until he was briefed before a raid on Dotcom's mansion in January.

He said yesterday that nothing had changed.

- © Fairfax NZ News