The gene also increases the risk of ovarian cancer and male breast and prostate cancer.
Mrs Coom, a mother of two and HR consultant, was 33 when she opted for the procedure, and started The Gift of Knowledge foundation in 2009 to provide education, support and solidarity for New Zealanders carrying the gene mutations.
"My mum had ovarian cancer and it was horrible watching her go through that so I knew when I tested positive that I would do everything I possibly could to avoid getting cancer and having my family go through it with me.
"I thought about it honestly for a few hours and I had made my mind up ... and if I was presented with the same choice to make tomorrow I would make exactly the same one."
A blood test will detect the gene, said Dr Mills. "The test means you are of increased risk, but it doesn't mean you have cancer."
People with direct relatives affected by cancer should consult geneticists and councillors before going ahead with tests, he said.
Tests were free if certain criteria were met, and up to $15,000 privately.
Breast Cancer in NZ
• About 2800 NZ women are diagnosed each year.
• More than 650 NZ women die.
• Genetic mutations have a 1 in 2 chance of being passed on to offspring.
• Education around signs, symptoms and regular mammograms is essential for early detection and increasing survival rates.
Source: New Zealand Breast Cancer Foundation