UPDATED 7.34pm A leading Green Party MP is set to swap the dispatch box for a role at a environmental charity.

Kevin Hague will be leaving politics and his role as a List MP in October to take over the reigns as chief executive at Forest and Bird.

Mr Hague, who lives on the West Coast, has been an MP since 2008.

He's also a former DHB Chief Executive, and has been regarded as the party's third ranked MP.

Mr Hague said his proudest achievement during his time in politics was as a result of one of New Zealand's worst industrial disasters.

"Probably the single biggest thing is major reform to New Zealand's health and safety legislation in the wake of the Pike River disaster."

Mr Hague said a gruelling travel schedule made worse by Air New Zealand's cuts to regional flights is one of the factors which prompted him to leave Parliament.

Mr Hague, who lives in Greymouth, said changes made this year by the national carrier meant he was away from home six nights a week.

"This is a brutal job," he said today. "It's really tough on families especially. And certainly for me, living on the West Coast, it's a hard place to get to and from.

"Air New Zealand has made changes this year that have made it harder," he said.

The airline halved the number of flights to Hokitika this year, he said, including the early and late flights.

"I've moved from probably being away from home, on average, four nights a week to six nights a week," Mr Hague said.

His partner Ian often travelled to Wellington for Parliament's sitting weeks, but had begun asking "what's the point?" because the MP often finished at work at 11pm.

Mr Hague said working at Forest and Bird would still be demanding but he would have "human-scale hours".

The MP, who entered Parliament in 2008, ran for the Green Party's male leadership position last year, losing out to James Shaw.

He was bitterly disappointed, but said today that he held no grudge against Mr Shaw and that his departure was not "a case of sour grapes".

Mr Hague admitted that he would have lead the party differently to Mr Shaw, but would not elaborate in detail.

"Probably more differences in style, I guess, and it's hard to be specific about those. He's such a suave character - I wouldn't have been as suave I guess."

Mr Hague is the party's third-ranked MP and its spokesman on health, conservation and rainbow issues.

In 2012, he was part of a group of MPs who led the fight to legalise same-sex marriage in New Zealand.

He successfully lobbied for reform of ACC, which was acknowledged today by the former ACC minister. In a rare moment of praise for the Greens, Judith Collins said she had always enjoyed working with Hague and that he was "always sensible".

As the Green caucus' strategic head, Mr Hague also played a role in the Greens' path to credibility over two election campaigns.

He worked to suppress the party's zanier aspects and banish anti-scientific policies such as opposition to 1080, fluoridation and immunisation.

More recently, he has been lobbying for the reinstatement of the Mental Health Commission, for adoption law reform, and the quashing of historic convictions for gay sex – issues which he hoped another MP would "pick up the baton” on.

Mr Hague would almost certainly have been a senior minister in a potential Labour-Greens Government, possibly taking on the health or ACC portfolios.

But the top job at Forest and Bird was worth more to him, he said.

"This was probably the only job that could tempt me away from my Parliamentary work."

Greens co-leader Metiria Turei said Mr Hague has made a huge contribution to the party and to the green movement.

"While we are sorry to lose Kevin from Parliament, at the same time, we are delighted Kevin will continue his contribution to the environment at Forest and Bird," she said.

"We know he will be a fantastic leader for this New Zealand icon."

Mr Hague's resignation means Barry Coates will come into Parliament as the next person on the Green Party list.

Mr Coates, a former executive director of Oxfam New Zealand, is at present working on trade issues at the University of Auckland Business School.

AIR NEW ZEALAND RESPONDS

In response to Mr Hague's criticism, Air New Zealand said this evening that it had progressively retired its loss-making, smaller Beech aircraft to move to a larger, 50-seat Q300.

Some regional centres did not have enough demand to support the larger aircraft, including Hokitika. As a result, there were now more seats but fewer flights from Hokitika - 14 return flights a week compared to 24 flights previously.

A spokeswoman said the airline paid careful attention to the West Coast community's needs, in particular the health professionals who flew in to work at Grey Base Hospital.

"However, we are sympathetic to MPs who have specific travel needs due to caucus times and long hours away from home and wish Mr Hague all the best in his next role."