Mike Sabin has resigned as a member of Parliament.
The Northland MP, who was also the chairman of the law and order select committee, said he had decided to resign "due to personal issues that were best dealt with outside Parliament".
There have been reports Mr Sabin was being investigated by police, although neither Mr Sabin, the police nor Prime Minister John Key would confirm that.
Mr Key continued to refuse to comment on when he first found out Mr Sabin's personal issues, but said today National had not asked Mr Sabin to step down.
He said his office was told Mr Sabin intended to resign yesterday.
"Sabin reached that conclusion himself on the back of personal and family reasons he is pursuing. My office was advised by Mr Sabin in the last day or so of his intention to resign.
"He's made that call to go and do that. He's obviously made the best decision for himself and his family."
He said Mr Sabin had been a good local MP and cited his anti-drug work as valuable for National.
"He made a very significant contribution to our caucus. He's a loss in terms of the contribution I've seen him make as a politician."
Mr Key said he was confident National would win the by-election in Northland and believed Mr Sabin was highly regarded in the electorate.
On Wednesday Mr Key said he was comfortable with Mr Sabin holding the role of chairman of the law and order select committee.
"I'm very happy for him to continue in the position he does," he told Radio New Zealand, adding that Mr Sabin was a "fully functioning member of the caucus".
Labour leader Andrew Little said Mr Sabin's departure so soon after a general election was unusual and Mr Key should have to answer questions about it.
"He might want to tell people what he knew [and] when he knew it ...".
He said Labour would stand in the byelection but conceded it had little chance of winning.
"We polled 16 per cent in the last election so Northland clearly is not a Labour stronghold. But they've got issues up there. They've got unemployment up round 8.6 per cent, one of the higher levels of unemployment in any country. They've got major problems with housing. And these are big issues for us. But it has been a National stronghold for a long time and I doubt whether that will change."
Labour's candidate last year was Far North District Councillor Willow Prime.
NZ First leader Winston Peters also questioned the Prime Minister's handling of the situation.
Mr Peters harks from Northland but would not say whether or not he was considering running in the byelection himself.
"This is not the plains of the Serengeti where we gather like animals at the first sign of a weak animal."
Mr Sabin had been National's MP for Northland since 2011 when he gave up his anti-methamphetamine consultancy business Methcon, which he formed following his departure from the police in 2006.
He lives at Coopers Beach with his partner.