Prime Minister John Key has confirmed he knew of former National MP Mike Sabin's personal matters last year, but says he had confidence in the MP up until his resignation.

Key rejected any suggestion he knew of the issues plaguing Sabin before the election.

"To the best of my recollection, the first that I knew about it was that last week of 2014, of the Parliamentary term," he told Radio New Zealand's Morning Report.

Key said he had "total" confidence in MP for Northland going into the election.

"And up until his resignation on Friday, I had enormous confidence [in him] as an MP," Key said.

"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 back in Government."

In a statement released on Friday, Sabin announced his departure saying he made the decision because of personal issues that were best dealt with outside Parliament. He would not comment further.

There have been allegations that he was at the centre of a police investigation.

His resignation will force a by-election in the Northland electorate just months after the general election. Sabin, a former policeman, is a second-term MP and was elected on an anti-methamphetamine platform.

Key said he had received advice from the Electoral Commission, and the date for the by-election would probably be in mid to late-March.

"There's two or three potential opportunities from the 14th to the 28th [of March]," the prime minister said

National was confident it would retain the seat, but Key said the party would be taking nothing for granted.

"Obviously you've got to work hard to win every by-election or every general election, but we've held that seat for a long time with John Carter, [and] we have obviously held it with Mike Sabin," he said.

"We've got a very strong policy record up there - we're in favour of continuing to develop both economic activities and the road access to the Far North.

"And if you look at Labour and the Greens they're vehemently opposed. That's one of the reasons I think they polled 16 per cent [in Northland] at the last election and that's not about to change."

Labour leader Andrew Little was realistic about the party's chances in Northland, but said the party would be using the platform to focus on what it saw as the big issues.

"The party machinery has to whip into action, once we know what the timeline is we'll start the selection process," he told Morning Report.

"And even though our polling in the Northland electorate doesn't give you a great deal of expectation we're going to win it, what it is is an opportunity to look at an electorate that is really suffering under a failed housing policy, a failed unemployment policy, where there is growing inequality, and we want to focus on those issues."