The Prime Minister is still refusing to answer any questions about when he found out National Party MP Mike Sabin may have been facing a police investigation.
The MP for Northland, Mike Sabin resigned from Parliament, today.
John Key said his office was informed yesterday.
In a statement, Mr Sabin said his resignation was due to personal issues best dealt with outside Parliament, but would make no further comment.
He had been under a cloud for some months, facing reports he was the subject of a police investigation.
While speaking to reporters in Auckland Mr Key, was asked if anyone from the National Party had pressured Mr Sabin to resign.
"No we didn't, Sabin reached that conclusion himself on the back of personal and family reasons he is pursuing."
Mr Key said he is confident National can retain the Northland seat in a by-election.
Earlier, Labour Party leader Andrew Little said there were still questions the Prime Minister should answer in light of the resignation
"He's lost an MP today and there is now a by-election and I think people are entitled to hear from the Prime Minister about that, his MP, and the taxpayers are going to go to the expense of a by- election.
"What he knew and when he knew and how to handle it...the reality is Mike Sabin has now resigned, there has to be a by-election, right now we're focused on preparing for that by-election."
New Zealand First leader Winston Peters agreed Mr Key should reveal what he knew, and when he first became aware of "matters" around Mr Sabin.
"Natural justice means we cannot ask for all the details of this case, but that does not preclude us from again questioning Mr Key and the National Party's handling of it.
"On December 28th New Zealand First requested an explanation from the Prime Minister and hitherto we have not got one.
"Only two days ago Mr Key told Radio New Zealand's Morning Report that Mr Sabin remained a fully functioning member of caucus.
"Mr Key was in full defence, so what has changed in two days?"
Mr Sabin entered Parliament in 2011, replacing the long serving National MP, John Carter as the MP for Northland.
At the time of his resignation he was serving as the chairman of Parliament's Law and Order select committee.
Mr Sabin joined the New Zealand police in the mid-1990s, and after a short posting in Auckland, returned to Northland to work as a detective in drug enforcement, in particular methamphetamine.
In 2006 he took leave without pay from the police, having founded MethCon, a company aimed at reducing the use of the drug P in New Zealand.
Mr Sabin's National Party biography said he and partner Sandra Mickell each had three children in their blended family.
They lived in the Doubtless Bay area. He attended Whangarei Boys' High School, and in the mid-1980s joined the Royal New Zealand Navy as a seaman officer.
Following that he worked in the dairy sector and became a sign writer.
Mr Key acknowledged Mr Sabin's resignation and also acknowledged his service as an MP since he was elected in 2011.
Mr Key is now taking advice on the possible date for a by-election in the Northland electorate.