Labour Leader Andrew Little says it is just not credible that the Prime Minister did not know about allegations concerning National's Northland MP Mike Sabin until late November or early December last year.
Mr Little today said he first heard about Mr Sabin's personal matters soon after he became Labour leader on November 18.
His office had received two pieces of information, independently of each other, that a National MP was under investigation. He subsequently alerted the Prime Minister's office in late November.
He said he made the decision to notify the Prime Minister's office and his chief of staff did so at the end of November.
Phone records obtained by Mr Little this afternoon show his chief of staff, Matt McCarten, rang Wayne Eagleson, John Key's chief of staff, on November 26.
"The issue here is that there was a serious allegation about a National MP. I considered that the leader of that party should investigate and, if necessary take action."
Mr Little said it was clear from the information received by his office that the allegations were not new. They had been around for some time.
"It is for that reason that I simply do not accept the Prime Minister's word that he did not know about the allegations until word was conveyed to his office."
Mr Little stopped short of saying the Prime Minister was aware of the allegations before last September's general election, or before Mr Sabin was appointed chairman of Parliament's law and order select committee in October.
"I simply don't accept the Prime Minister's word that he did not know about the allegations before they were conveyed to his office from my office.
"In our little democracy here the Prime Minister pretty much knows everything and the Police Minister is briefed by his department [police department] on sensitive issues so I just do not accept senior ministers of the Government do not know about this."
Earlier in the day, the Prime Minister said Mr Little was mistaken and that Matt McCarten had contacted Mr Eagleson about the allegations on December 3 - although Mr Eagleson knew about them already.
Mr Key said he himself had found out on December 1.
"We've never denied that my chief of staff knew in late November. I was told in very early December."
Mr Key said it would have been "useful" if Mr Sabin had told the party about the personal matter earlier "but he didn't and we didn't know about it".
Mr Sabin resigned as an MP last Friday "due to personal issues that were best dealt with outside Parliament".
A by-election will be held on March 28.
When asked if he was one who first alerted the Prime Minister's office about the allegations, Mr Little said "I'd be very surprised if I was the one. It was clear to me that the allegations had been swirling around for a while."
When asked what Mr Key's motivation would be for saying he did not find out until December, he said it was possible Mr Key was trying to diminish his level of knowledge.
He said the Prime Minister's office acknowledged receiving his note but that was the last he heard about the matter.
Mr Key's office said on Monday that Mr Eagleson was made aware of the issue in late November and told Mr Key the next week. They were unable to clarify whether Mr Sabin had told Mr Eagleson directly.