Prime Minister John Key has continued to refuse to comment on when he first found out Mr Sabin's personal issues, but said 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 theback 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.
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."
