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Callaghan, a mother of two children, was 36 years old.
She died in Wanaka on Thursday surrounded by loves ones and family and without pain, her friend Emma Simpson said on social media.
"I know this news is utterly devastating," Simpson wrote.
"I know how heart-wrenching it is that someone we all love, as well as a stunning mother of two that fought so bloody hard, could be taken so early. I also know there's a hell of a lot of cliches that could be said here.
"But that's not our Kate's style. She fought, hard. Bloody hard. And her many, many victories, her impact on others and the legacy of knowledge and successes she has left behind are cause for celebration."
Callaghan was described as a "ball of resilience, an absolute knowledge fiend, beyond empathetic, loving to the core and above all, a fighter".
Callaghan first felt a lump in her breast last June, Stuff reported.
Her healthy lifestyle led doctors to believe she had a very low chance of having breast cancer and she was not pushed to be tested, she said last year.
A breast surgeon at Dunedin Hospital also thought it was unlikely she had cancer.
But in November, scans showed she had stage four breast cancer and that it had spread to her lymph nodes and liver. That meant she was not eligible for surgery and could only get palliative chemotherapy or radiation therapy.
"This is not right," Callaghan said on Instagram at the time. "This is not how it's supposed to be. I'm a healthy, 35yr old mum of 2. I'm not done here yet."
Simpson said that after getting her diagnosis, Callaghan "bought the biggest, heaviest, peer-reviewed book on treatment and ravaged through it".
She named her cancer "Sharyn" and would "verbally tirade against her".
Earlier this year, Callaghan travelled to Mexico for alternative cancer treatment at a centre called Hope4Cancer, after crowdfunding for the trip.
She came back home before New Zealand went into Covid-19 lockdown. After getting radiation therapy and trying further medications, she told doctors she no longer wanted a prognosis, Stuff reported.
"I feel good at the moment – I feel really good. I don't see myself dying anytime soon."
Callaghan described herself as a holistic nutritionist and was an author and blogger with a large following online.
She leaves behind her husband, Aaron, and two children Olivia, 5, and Ed, 3.