Prime Minister John Key has said he only became aware in December of the personal reasons behind Mike Sabin's sudden resignation as Northland MP.

Speaking on Radio New Zealand's Morning Report today, Mr Key said he did not have "any great information about those personal and family matters prior to that".

Mr Sabin resigned as an MP and chairman of the law and order select committee on Friday "due to personal issues that were best dealt with outside Parliament".

There have been claims Mr Sabin was being investigated by police, although neither Mr Sabin nor the police would confirm that.

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Mr Key said on Friday that National had not asked Mr Sabin to step down and this morning he described him as a "good, hard-working local MP".

"Up until his resignation on Friday I've had enormous confidence in him as an MP.

"Actually I think he has performed very strongly, he's got a great grasp of some of the policy area, I actually saw him as someone that would be potentially a future cabinet minister if we were in government."

He said he was aware Mr Sabin was planning on resigning one to two days before it happened.

"I was aware that he was leaving for personal and family reasons and making his resignation late last week, a couple of days before it happened," he said.

"In terms of those personal and family matters, I became aware of those at about the last week of the parliamentary term last year.

"I didn't have any great information about those personal and family matters prior to that, and even when I was aware of those, I'm often aware of personal and family matters raised in the context of someone saying their spouse is sick, or they've got a particular test that they're going in for...it wouldn't have been something that would have driven an outcome from me.

Mr Key said a by-election in the Northland electorate was likely to be held in mid-to-late March.

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He was confident National would be able to hold onto the seat.

"We've held that seat for a very long period of time," he said.

"If you look in the Far North, John Carter's held the seat for a long time prior to Mike Sabin. Mike Sabin was a good, hard-working local MP. The majority was over 9000."