Rizvi was diagnosed with a rare brain tumour at 31. Eight years on she navigates complex disabilities due to her acquired brain injury.
Waterland, 38, has spent much of her adult life managing complex mental health battles in the aftermath of trauma from childhood abuse and neglect.
The pair, who have been friends for years, quickly discovered their physical and mental illnesses had a similar mortality rate and even some of the same symptoms.
Yet while Rizvi received countless lasagnes and offers of help following her diagnosis, Waterland said her fridge stayed empty.
"We definitely knew at that moment that there was a book to share and that there was something worth exploring there," Rizvi told AAP.
"It felt like it was about something bigger than myself, something bigger than Rosie as well ... and together we're able to paint quite a universal picture of what it looks like to be unwell and how we treat different illnesses on a hierarchy of importance."
The book Broken Brains comes out on Tuesday, the title in part taken from an expression coined by Rizvi's son Rafi.
"(It's) a phrase he concocted when 'broken' was the very worst thing that could happen to someone - but importantly, something that could also be fixed," Rizvi writes.
The publication has been a long time coming and the pair are relieved it has eventuated, four years after their original deadline.
"Jamila had a regrowth of her tumour at one point, so we had to pause for that. I had a pretty bad mental health relapse and we had to pause for that," Waterland said.
"The very situation of having to delay this book because we were unwell taught us more about how we live with our illnesses and the reality for both of us of living with the conditions we have.
"The extra time we got was actually a blessing, because the book is much better for it."
Rather than being a grim depiction of living with illness, Broken Brains offers insight, humour, comfort and hope to anyone who has faced a physical or mental illness, and those who care for them.
Both women are successful authors and commentators, each having previously published multiple best-selling books.
In Broken Brains they share deeply personal stories, while weaving in the expertise of medical professionals and experiences of Australians who are living with various illnesses.
They also explore the difficulties in getting a diagnosis, particularly the challenges women face to have their pain believed.
"Throughout history, men and women have been treated very differently by the medical profession," Rizvi said.
"There is still stigma where women feel disbelieved and where women's pain is taken less seriously: women feel more self conscious about expressing pain and women are more likely to underrate their pain."
Ahead of a month-long book tour, Waterland and Rizvi are prepared to hear the personal stories of many of their readers.
At a book signing following their first event at the Sydney Opera House on Sunday, the pair stayed for more than two hours chatting to people who had lined up to meet them.
Waterland, who has written extensively about her mental health in two memoirs, knew Broken Brains readers would want to share their own stories.
"When people share their experiences with you, it's because you've succeeded in what you wanted to do," Waterland said.
"It's a really beautiful thing, because it means that what you've written has made them feel seen, and that's why you do it."
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