Background: Breast cancer is an important cause of illness and death in New Zealand women. It is the most commonly registered cancer in women, and the second most common cause of cancer death in women. In 2009, 2,759 women were diagnosed with breast cancer and 658 women died from breast cancer. The incidence of breast cancer is projected to stabilise in New Zealand, but the actual number of women diagnosed with breast cancer will increase because of the increasing proportion of older people and increasing size of the population. Methods: Estimates of the magnitude of modifiable lifestyle risk factors for breast cancer (relative risks and odds ratios obtained from published epidemiological studies) and the prevalence of exposure in New Zealand were used to calculate the population attributable risk percent (PAR%) for each risk factor. The PAR%s show the relative importance of these risk factors, and give an indication of the potential impact of reducing the prevalence of these lifestyle risk factors on the incidence of breast cancer in New Zealand. Results: Six modifiable lifestyle factors were identified for breast cancer. These were obesity, lack of physical activity, high alcohol intake, oral contraceptive use, hormone replacement therapy (HRT), and delayed first birth. The PAR%s for these risk factors ranged from 1% for delayed first birth to 10% for obesity (16% for Maori women and 17% for Pacific women). Conclusions: The most important primary preventive strategies to reduce the risk of breast cancer in New Zealand are promoting lifestyle changes to reduce obesity, promoting regular physical activity (which may in turn reduce the prevalence of obesity), reducing HRT use, and avoiding high alcohol intake. A strategy to promote regular physical activity and reduce obesity could also have other benefits, such as reduced risks of cardiovascular disease and diabetes.