Prime Minister John Key has announced he is to quit his position after eight years, saying that he owes it to his family to step aside.

He told the media that he won't be seeking a fourth term as leader, saying he had "given everything I could for this job" and that he will be leaving politics, possibly seeking roles on boards in Australia, Asia or New Zealand.

Mr Key said spending time with his family was a major factor in his decision to resign as Prime Minister and as National Party leader.

"Ten years at the top is a long time," he said.

He said a "lot of leaders stay at the top too long."

"Throughout these years I've given everything to this job - a job that I cherish in a country that I love," he said.

"All of this has come at quite some sacrifice for the people that are dearest to me, my family."

Mr Key said there had been many night and weekends that his wife Bronagh had spent alone and that "I really feel I owe it to the family to come home a little bit".

"My daughter Stephi and my son Max transitioned from teenagers to young adults while coping with an extraordinary amount of intrusion and pressure because of their father's job," he said.

"I thank them for their tolerance, Bronagh and I are immensely proud of them.

"My family has also had remarkable opportunities and experiences as we have met people and travelled from one end of the country to the other."

What happens now?

Mr Key said he expects that on Monday, December 12 there will be a special caucus meeting to select a new National Party leader.

He will then tender his resignation as Prime Minister to the Governor General.

Mr Key said it was inevitable that he will be asked who he will vote for at the caucus meeting.

"Whoever the Caucus selects will have my unwavering support, but if Bill English puts his name forward I will vote for him."