Former Green Party co-leader Jeanette Fitzsimons has died.

Her husband, Harry Parke, said the death was totally unexpected.

"Yesterday morning she was out on the farm doing stuff, she had a bit of a fall and finally ended up in Thames Hospital where she had a massive stroke and died at 9.45pm last night - very peacefully I might add."

"The day before, she was using a chainsaw - that's the sort of person she is. She worked a lot harder than I ever did. I was totally in awe of her.

"Fortunately we both had very much the same convictions about what needed changing in the planet and we had a very close relationship."

Fitzsimons became the co-leader of the Green Party of Aotearoa New Zealand in 1995, and when the party joined the Alliance led by Jim Anderton's New Labour Party, she took on the deputy leadership role.

After the first MMP election in 1996, she entered Parliament as a list MP for the Alliance but it wasn't long before strains appeared in the grouping.

She felt herself left out of its decision-making and the Green Party itself was increasingly unhappy with the Alliance's direction.

The agreement to send New Zealand troops to Afghanistan in the United States'-led so-called war on terror was a step too far for the Greens and they left the Alliance.

Fitzsimons won the Coromandel seat for the Greens in 1999, the country's first elected Green MP and was disappointed when she lost it in the following election, although the party remained in Parliament due to its party vote.

She and her co-leader Rod Donald were strong influences in the change in public perception of the party as a group of sandal-wearing tree-huggers.

Parke said Fitzsimons was "instrumental" in getting the Green Party up and running in the 1990s. More recently, her focus had been on climate change.

"She fought really hard to get people to accept you can't keep growing the economy and stop climate change. It just seems people don't want to hear that."

Fitzsimons was known for never raising her voice in the House and never responding to barbs thrown around in Parliament.

"She strongly believed that never got you anywhere, that all it did was take the focus off the subject you were talking about and your energy needed to be totally on what you were trying to achieve. I think she held that up admirably," Parke said.

"She never let her emotions get in the way of what needed to be said and what needed to be done."

"She totally lived her convictions and there was no way that anyone could say she didn't live up to what she was saying."

In 2018, she addressed the Green Party's two-day annual general meeting, and later told reporters what the Green Party had achieved after 10 months in Government was impressive but the caucus should be careful not to get the "speed wobbles".

"James' [Shaw's] account of what has been achieved by three and a half ministers in Government is pretty impressive and I'm delighted by the portfolios we got and the work the ministers are doing," she said.

"There's always the risk at this stage of getting the speed wobbles. There's always a tension between keeping the Government going smoothly and getting a lot of achievements from ministerial work and staying true to our original kaupapa."

But Fitzsimons said the Greens' support for the Electoral (Integrity) Amendment Bill was a low point.

"It's wrong, it denies MPs' basic freedom of speech, freedom of association, freedom of thought. It's contrary to the Bill of Rights and its contrary to our policy."

Fellow former Green Party MP Mojo Mathers tweeted this morning: "My heart is breaking, and the tears are falling. Jeanette, thank you so much for everything you have done, for standing so staunch and strong for the planet. We will miss you so much."

Deputy Prime Minister Winston Peters said on Twitter he was "very sad" to hear of Fitzsimons' death. "She was someone who had her principles and stuck to them."

Fitzsimons was made a Companion of the New Zealand Order of Merit in 2010.

By RNZ and with NZ Herald reporters