Updated 4.26pm: According to the Prime Minister his Northland MP Mike Sabin jumped ship on his own accord.

John Key says the party didn't ask for Mr Sabin to resign.

"No we didn't, Sabin reached that conclusion himself on the back of family and personal reasons."

There's little praise for departing National MP Mike Sabin from his own party.

He's announced his resignation today.

It follows weeks of speculation about his future as neither he, the police, nor the Prime Minister would confirm if the Northland MP was subject to a police investigation.

John Key's acknowledged Mr Sabin's resignation and now National Party President Goodfellow is doing the same.

However, neither man has offered a single word of commendation to Mr Sabin.

Cabinet Minister Simon Bridges says he has no knowledge of the details that led to Mr Sabin's decision.

"Obviously it's saddening from a number of perspectives, and it's obviously a significant matter when a Member of Parliament, a relatively new one, resigns."

In a statement, Mr Sabin said he had decided to resign due to personal issues that were best dealt with outside Parliament.

Just two days ago, John Key said he was happy for Mike Sabin to continue as chair of Parliament's law and order select committee - amid pressure for him to stand aside from that.

The Labour leader's refusing to play dirty - after National MP Mike Sabin today resigned, forcing a by-election in Northland.

Labour's Andrew Little says Mr Key will have questions to answer, but his own party's got other priorities.

"The PM obviously knew something at that time, and people can judge his responses at the time. We've now got the resignation, that hasn't come out of the blue, but Labour just has to get up and ready for the by-election."

"Stepping an MP aside, especially when that MP has responsibility as Sabin had for the law and order committee, that might have been something that ought to have happened earlier."

New Zealand First leader Winston Peters says it's time for the Prime Minister to explain what he knew and when did he know what was going on with his MP.

John Key's coy on what he knew and when - and he's unsure what disclosure Mr Sabin would have made to the party, before being re-selected as its candidate.

He referred questions to party president Peter Goodfellow - who is not returning calls.

A spokesman for National has confirmed that election nominees are expected to disclose such information to the party.

Mr Peters says natural justice means the reason can't be demanded but National's handling of it can.

He says John Key was saying only two days ago that Mr Sabin remained a fully functioning member of National's caucus.

Mr Peters wants to know what's happened since then.