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.