But landmark research has highlighted the likely true extent of their ranks for the first time, pointing to dramatic underestimates in official figures.
Almost 8000 people, mostly women, are now thought to be living with the incurable, advanced form of breast cancer in NSW alone, triggering calls for the new method of counting them be adopted nationwide.
Andrea Smith was blindsided when diagnosed with the condition in 2016 after the cancer had spread to her bones and liver.
Andrea Smith says people with metastatic breast cancer need more support. (Steven Markham/AAP PHOTOS)
She was weeks away from submitting her PhD on tobacco control, but the diagnosis shifted her focus in a new direction.
Dr Smith is now a researcher at the University of Sydney's Daffodil Centre, focusing on survivorship and supportive care in the absence of a cure.
"We will all eventually die from this disease once we run out of treatment options," she told AAP.
Like one in 20 people diagnosed, she had no prior experience with breast cancer treatment before learning she had an advanced and incurable form of it.
"I was quite surprised that I was being treated differently, like there were breast cancer nurses at my hospital, but they only supported people with early breast cancer," Dr Smith said.
Advocates launched the nation's first specific advocacy organisation, Metastatic Breast Cancer Action Australia, on Wednesday.
The launch coincided with the release of fresh research about how many people were living with metastatic breast cancer, as evolving treatments help them do it for longer.
The NSW Cancer Institute linked together data from multiple sources to paint an accurate picture of the disease's prevalence in an analysis described as world-first.
The institute's chief executive, Tracey O'Brien, said the data would help support disease management.
"Although a cure for metastatic breast cancer remains the ultimate goal, our immediate focus must be on helping those living longer with the disease to live well," she said.
Support for people with breast cancer mostly caters to people at an earlier, curable stage. (Lukas Coch/AAP PHOTOS)
Breast Cancer Network Australia said national estimates of about 10,500 people living with the disease were inaccurate given the research showed there were 7850 women and 50 men in NSW alone.
The network's policy director, Vicki Durston, called for federal government support for other states and territories to adopt the NSW methodology.
"It has delivered a model that will pave the way for other cancers to be counted and for other states to follow," she said.
Health Minister Mark Butler said the Cancer Institute's data collection was world-leading, indicating it would be adopted across the nation.
"Knowledge is power and this information will help drive a new era of research to improve the lives of thousands of people, not just in NSW but across the country," he said.
Dr Smith said properly understanding how many people lived with metastatic breast cancer was vital to providing support.
"If we don't know how big the population is, it's extremely difficult to advocate for appropriate services ... which is why people with metastatic breast cancer often report feeling invisible or sidelined," she said.
Support for people with breast cancer generally catered to people at an earlier, curable stage of the disease, Dr Smith said.
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