UPDATED 2.55PM New Zealand unions are grappling with the news of Helen Kelly's passing.

LISTEN ABOVE: Labour leader Andrew Little spoke with Chris Lynch about Helen Kelly's passing

Kelly, 52, the former Council of Trade Unions president of eight years, was diagnosed with terminal cancer in February 2015.

Former Labour Prime Minister and head of the United Nations Development Programme Helen Clark said she was "deeply saddened" by news of Kelly's death.

"Helen was a lifelong and very effective advocate for workers' rights," Clark said.

"She has fought a very challenging illness with the courage which has characterized her entire life, and continued her advocacy for others even while very ill herself.

"Helen was a truly remarkable person, and will be greatly missed. I offer my deepest condolences to her family and her very large community of friends."

Prime Minister John Key tweeted this morning: "Saddened to hear about the passing of Helen Kelly, a strong advocate for workers' rights.

"My thoughts are with her family and friends."

Saddened to hear about the passing of Helen Kelly, a strong advocate for workers’ rights. My thoughts are with her family and friends.

— John Key (@johnkeypm) October 13, 2016

A close colleague of Kelly's for decades and current CTU President Richard Wagstaff said she changed New Zealand for the better and her work will continue after she's gone.

"She saw there was much to be done to make life better for working people and she wasn't afraid to take on the real challenges and vested interests that were a barrier to achieving a better world for everybody and for those who're particularly not getting a fair deal in New Zealand."

He said the next few days will be a tough passage for everyone and it's a very sad day for the whole New Zealand union movement and they'll miss her deeply.

“Even though we’ve known this is coming it’s still a terrible shock to be confronted with the realization that we won’t be speaking with her again and sharing her company so we’re trying to get our heads around that.

“She is a remarkable woman, a wonderful inspiration to working people and she left a wonderful legacy to go on with.”

Wagstaff said unions are inspired by and will continue her work for a long time to come.

Although she stood down from the CTU in October last year, Kelly continued to fight for the causes she believed in until the end.

She fought for tougher safety regulations in the wake of the Pike River mine disaster and oversaw the union bringing private prosecutions over the death of forestry workers after the Labour Department had failed to lay charges.

Kelly also advocated for pay equality and employment equity and was involved in a major stoush with the government and filmmaker Sir Peter Jackson on rights for those working on The Hobbit movies in New Zealand.

More recently that she fought for the right to use medicinal cannabis.

Labour leader Andrew Little said she was a passionate advocate for working Kiwis.

"Her tireless campaigning and advocacy made a real difference and her commitment to her cause saw her championing the need for safer working conditions for those in the forestry industry, shining a light on an industry that has for too long neglected the people who work in it.

"To the very end of her life, she was an example of extraordinary dedication and tenacity.

"Helen spent her life standing up to powerful people and fighting for justice. Her passing is an enormous loss to New Zealand," he said.

Labour's finance spokesman Grant Robertson said he's never met a person with deeper, stronger principles which she applied to everything she did.

He's praising her courage and fortitude, but most particularly her kindness.

"And I think about the women of Pike River, the families of forestry workers that were killed, the kindness that she showed towards them is something all of us could aspire to."

Business New Zealand Chief Executive Kirk Hope said Helen Kelly was challenging, but also a delightful mischief to work with.

Hope said Ms Kelly had made a significant contribution to public life and to the interests of working people in New Zealand.

"And although we were often on opposing sides of the table there was always great respect for Helen's work... she was a constructive, well-organised, passionate and hard working advocate who will be greatly missed."

Grey District Mayor Tony Kokshoorn worked with Helen Kelly during the Pike River Mine Disaster.

He said Helen was always helping out grieving families and would go to the memorials every year.

“She was the type of person who had an unbelievable temperament, her personality was second to none and she always showed great concern for others that really puts her out in front.

“She just had that personality where she just reached out to people and it was not overwhelming and you always had your say with her she was a great listener who believed deeply in the people of New Zealand.”

In her honour, the families of Pike River miners and forestry workers killed on the job set up a support group for other families.

Former MP Sue Bradford said Helen Kelly worked strongly for the union movement her entire life.

"Feeling very sad about the news that Helen Kelly has died. A woman that many thousands of people in the union and community activist groups around New Zealand really we've been inspired for workers who've died or been injured on the job.

"I think she brought something really special to the role and I know she's touched the hearts of many people, both union members and their families throughout the country, for the work she's done. I'm feeling very sad today."

Fellow medicinal cannabis supporter Huhana Hickey said Kelly was a compassionate person who kept fighting for people until her last breath.

“I just immediately felt the sadness that our whole country is going to feel, she dedicated her life to people, she dedicated her life to the workers.

“She dedicated her life to change and social justice, we haven’t got leaders like that in large numbers here and we’re going to lose a significant voice in this country,” she said.

Dr Hickey said Kelly advocated for people because she cared about struggling people.

Kelly's involvement with unions began at an early age.

Her father was Pat Kelly, a staunch and well-known unionist widely regarded as the intended target of the 1984 Trades Hall bombing in Wellington.

Despite struggling with pain, in August she told The Nation she had had a wonderful year and would not want to have spent it simply lounging around.

Kelly leaves husband Steve Hurring and son Dylan.

RIP Helen Kelly. Your courage, warmth and strength were a huge inspiration for me. Thank you for all that you did for others.

— Mojo Mathers (@mojomathers) October 13, 2016