Prime Minister John Key has hinted at some frustration with former MP Mike Sabin for failing to tell National about the personal issues that led to his resignation prior to the election, saying Mr Sabin had almost been appointed as a minister and news of his issue had come as "a shock."

Northland voters will go back to the polls for a byelection on March 28 after Mr Sabin resigned last week citing personal issues.

That came a month after reports he was being investigated by police.

Mr Key said the first he knew of any problems Mr Sabin faced was in early December when his chief of staff told him. It is understood Mr Sabin's issue arose prior to the election. Mr Key revealed Mr Sabin was on the cusp of being appointed as a minister when National was re-elected.

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"To be frank, he was on the list of 'likely to be a minister.' It was a real toss-up between him and a couple of other people who got in. That's how confident we were, or how lacking in knowledge of other issues we were. So it came as quite a shock to me when I was told of the matters he was pursuing."

He said MPs were encouraged to let the leadership know of issues early on but different people reacted in different ways. "In the perfect world we live in a world of no surprises and they tell us as early as they can if they have a particular family matter. Some are better than others at telling us straight away."

Mr Key said he may have stood Mr Sabin down in early December if he had been a minister but at that point did not believe it was necessary to to do so for a select committee chair.

"The reason I didn't was because I thought on balance, given the information I had at that time, the course of action I took which was to let things ride and see how they progressed was about the right course of action. If he was a minister I probably would have taken the step of standing someone down, but as a select committee chair, I don't think so."

Mr Key said the National Party President, Peter Goodfellow, was also unaware of Mr Sabin's issue prior to the election. He did not know whether any Government ministers had known earlier under the 'no surprises' policy the Government operates.

National Party President Peter Goodfellow said although the electorate had long been held by National there was usually a much lower turnout for byelections.

"We will not be taking Northland for granted and my message to the successful candidate will be: prepare to put in the hard yards, we've got a big job ahead."

He said nominations to be National's candidate will close on February 12 and the selection will be made at the end of the month.

Mr Key said part of the reason for the lengthy selection period was to ensure National picked a good candidate. "That's a seat we're very keen to retain."

Labour leader Andrew Little said Labour will stand in the byelection although his hopes of winning were slim given Mr Sabin's 9000 vote majority. Willow Jean Prime was Labour's candidate in 2014 and the front runner to stand again. She is on the Far North District Council and re-selecting her would have the advantage of being able to re-use her election advertising material.