Nicola Coom says if she had to make the decision again, she would still "happily" have a double mastectomy.
The 37-year-old Christchurch woman is one of thousands of New Zealanders who carry genetic mutations BRCA1 and BRCA2 that increase their risk of breast cancer by five to eight times.
The genes are tumour suppressants which increase the risk of a tumour developing when genetic faults occur, said Dr Stephen Mills, New Zealand Breast Cancer Foundation medical advisory committee chairman .
"The BRCA genes prevent cells behaving abnormally, so when you have the mutation then there is an increased risk that these cells in certain parts of the body behave abnormally so can cause cancers."
Carriers of the genes have a 50 to 85 per cent risk of developing breast cancer compared to the general population risk of about 10 per cent, and there is a 50 per cent chance it will be passed on to their offspring.
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