Former Green Party co-leader Russel Norman has quit the party before beginning his new job as executive director of Greenpeace New Zealand on Monday.
His career as an MP officially ended at midnight last night.
Dr Norman, who was co-leader of the Green Party from 2006 to earlier this year, said he was pleased he had left the party in a strong position.
But starting a new job, he could not remain a member of the party, he said.
"It's very important to Greenpeace that it's politically independent and that it's seen to be politically independent and so that's why it's so important I think that I do resign my membership of the Green Party.
"Obviously wish the Green Party well but, you know, in Greenpeace we've got to have an independent voice."
Dr Norman said he would be lobbying all political parties, including those he opposed in Parliament, in his new role.
But he said his main focus would be on trying to galvanise people to take action on the environment and climate change.
"In Greenpeace we're very focussed on the community and winning people to the ideas of sustainability, taking action around the ideas of sustainability, and in time that will force governments and political parties and business to act on those things."
Dr Norman was elected to Parliament in June 2008.
In his maiden speech, he said he was not ashamed to say he had a history in the socialist movement.
"I am not ashamed to think that everyone deserves a fair go and a fair share of what this life has to offer; to believe that this world is too divided.
"But nor do I shy away from my reasons for embracing Green politics, for moving on. The two great failings of socialism, as I knew it, were a lack of democracy and a failure to grasp the reality that the life-support system of the planet itself is endangered."
He had first got involved in Green politics as part of a campaign to stop logging of forests in Gippsland, Victoria, but came to live in New Zealand in 1997 and joined the Green Party.
"There is perhaps a single new idea at the heart of the Green movement, and that is that the planet is finite.
"The planet has limited ability to absorb our pollution, and supply us with resources. It has limited minerals, forests, soils, rivers, and lakes.
"The realisation that the planet is finite is a simple, yet profound, insight into the true nature of human existence."
People had to be told about the state of the world and that greenhouse gases had to be cut, he said.
Whether representing Greenpeace or the Green Party, his message will not change.