It's day two of the Labour leadership battle and Prime Minister John Key has already accused one of the front-runners of lying.



New Lynn MP David Cunliffe this morning ruled out making Greens co-leader Russel Norman his finance minister in any post-election coalition, but Key said Cunliffe was lying and would do anything to be prime minister.



"If, in the end, the price of having a government with the Greens is Russel Norman being the deputy prime minister and minster of finance, will he rule that out categorically? The answer will be no," Key said.



Asked if he was accusing Cunliffe of not telling the truth, Key replied: "He's lying to you.



"In the end he will be desperate to become prime minister, in the end he will do whatever it takes and say whatever it takes.



"Come the moment that's the price, he'll say 'well I'm doing my bit for the country'."



Norman has made no secret of the fact that he would expect a top job in a Labour-Greens government and Cunliffe said this morning that under his leadership Norman could expect a senior economic role.



Key insisted, meanwhile, he would be "transparent" if National reversed its position on NZ First. He ruled out working with leader Winston Peters before the 2008 and 2011 elections.



"If we change our position and neutralise Winston Peters and I'm prepared to have a discussion with him, I'll also make that clear to the New Zealand public," Key said.



Since David Shearer quit as Labour leader last Thursday, Key has been pushing the message that a leadership change will move Labour further to the left.



He said this morning he "doesn't care" who his new opponent was.



Cunliffe shot back at Key's lying claims with an accusation of his own.



"I would say that Mr Key ought not to make statements about other people telling lies ... because his record might not be entirely clean - ask him about GST for example," he said.



He reiterated Norman would not be finance minister in any government he led, but Labour rival Grant Robertson would get a top job.



"There's been an allegation that I or my team have been promising jobs to people in return for support, I just want to say right up front that that's absolutely untrue," he added.