Updated 5.28pm: The Prime Minister's put a dampener on speculation National's been casting around for weeks, for a new candidate in Northland.

John Key today announced a by-election will be held on March 28th and rejected rumours that National's been scouting for a replacement for some time.

"We're going to open nominations and it's going to take a good four week period, and that's because we want to secure a good candidate for a seat that we want to retain so no, that's not true."

Candidates will have until March 3rd to get their names in.

John Key is confident National will retain the seat.

"We've held that seat for an awfully long period of time with John Carter and then with Mike Sabin.

"We are proponents of building better road access and improving the road up to Northland."

Labour's already got a candidate in mind for the Northland by-election - just days after National MP Mike Sabin resigned.

Labour's general election candidate, Willow-Jean Prime, has already announced her intention to stand.

She's got the backing of leader Andrew Little, well ahead of Labour opening its nominations.

"I'd be very keen for her to be that candidate, she performed very well. We'll go through our process and see what it comes up with, I'd be very surprised if it wasn't Willow-Jean Prime."

John Key isn't saying much more about why Mike Sabin has resigned...venturing it was the MP's choice to leave parliament and that he wasn't pressured to do so.

Meanwhile, the Prime Minister is confident about his handling of the resignation of his former MP, Mike Sabin.

Winston Peters says John Key has questions to answer about when he was informed of those matters.

But the Prime Minister says he's got nothing to worry about.

"I mean, I became aware of those personal and family matters at the very latter part of the 2014 parliamentary year.

"It might have been the last week of Parliament."