The Prime Minister now says he found out about personal issues that would lead to the resignation of National MP Mike Sabin a week earlier than he previously said.

Photo: RNZ / Jane Patterson

Mr Key originally said he became aware of those matters during the second week of December.

Labour Party leader Andrew Little told Morning Report today his staff learned of those issues in late November last year, and told John Key's office straight away.

This morning, on his way into an all-day caucus at Premier House in Wellington, Mr Key said he found out on 1 December, and the phone call from Labour came two days later, on 3 December. Independent of the phone calls, his Chief of Staff, Wayne Eagleson, knew in late November.

"My understanding is the Chiefs of Staff had a discussion this morning, and I don't think it's disputed that they had a conversation alright, they had it in early December.

"But we've never denied that my Chief of Staff knew in late November."

The prime minister was asked how Mr Eagleson found out in late November.

"There were a number of sources, it's my understanding," said Mr Key. "And in fact the phone call from Andrew Little's office, from Matt McCarten, was really to say they had heard some rumours and we were aware of it.

"They didn't see it as a political issue, they were just making sure that we knew."

Party President Peter Goodfellow said he would always expect potential National Party candidates to inform the party before an election if they were facing any issues that could affect them politically.

"Mike Sabin was selected in December 2013, so I don't know whether he knew then or not."

Mr Goodfellow said he found out in late November or early December but did not know the specific date, and would not say who passed on the information.

He said he was told there was a police investigation, but did not pass that on to Mr Eagleson because "I knew that he knew."

The by-election for the Northland electorate, prompted by Mr Sabin's resignation, will be held on Saturday 28 March. Mr Sabin said he was resigning due to personal issues but refused any further comment.