Northland by-election likely to be in March

Mike Sabin.

A by-election in the Northland electorate is likely to be held in March, following the resignation of National MP Mike Sabin last week.

Mr Sabin announced his resignation on Friday, citing personal issues that were best dealt with outside Parliament. He would not make any further comments.

His resignation came a month after reports he was being investigated by the police.

Speaking on Radio New Zealand's Morning Report this morning, Prime Minister John Key said following advise from the Electoral Commission, the by-election would be held in mid to late March.

Mr Key said 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."

Mr Key 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.

"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.

"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."

Mr Sabin was previously chairman of Parliament's law and order select committee.