Ms Lashlie was hospitalised after Christmas when a scan revealed pancreatic cancer.
Yesterday in a statement, her family said her condition had deteriorated significantly in the last six weeks.
Yesterday Ms Lashlie said on her website that she had become unwell last year.
"Late last year I slowly became unwell. The stress of the lifestyle I was living, the demands I made of myself, the demands other people made of me and expected to meet became too great and as 2014 closed I was diagnosed with pancreatic cancer that had spread to my liver," she wrote on her website.
"No treatment, no cure, only palliative care. I'd waited too long to look after myself and my body broke.
"It's time to leave the work to others now."
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Big Buddy chief executive Richard Aston, whose organisation mentors fatherless boys, said Ms Lashlie's death would be "sorely felt" throughout New Zealand.
"Celia was a fantastically authentic woman with a fierce intelligence, a big heart and a great storyteller.
"She could lower her head and look at you with eyes that could melt icebergs and just as quickly roll her head back laughing. Funny and serious, she could laugh at herself."
Meanwhile, a Givealittle page has been set up to honour the work of Ms Lashlie and support the projects nearest to her heart.
"A special group of Celia's family and friends will use the funds to continue Celia's work, in accordance with her wishes," the Givealittle page said.
"The idea is that we can progress some of the work Celia intended to do, before she was diagnosed with the horrible disease that is pancreatic cancer."
The page, which was created yesterday, has $1385 in donations so far.
Ms Lashlie is best known for her work with the 'Good Man' project that focused on research from discussions with pupils in 25 boys' schools throughout the country.
It formed the basis of her book, He'll Be OK - Growing Gorgeous Boys into Good Men.
She has also written two other books - The Journey to Prison: Who Goes and Why, and The Power of Mothers: Releasing Our Children.
Her advice to mothers of boys became well known, and her work on raising teenage boys as well as on social justice issues saw her in demand for speaking in New Zealand, Australia, South Africa and the United States.
Ms Lashlie was also the former manager of Christchurch's Women's Prison, and worked for 15 years within the Prison Service from 1985.
She was the first woman to work as a prison officer in a male prison in New Zealand.
Ms Lashlie was the mother of two adult children and 'Nana' to five grandchildren.
Lashlie was a New Zealand Herald New Zealander of the Year in 2001.
