A long-term study has provided clinicians with "enormous power" to assess the cancer risk of women carrying the BRCA1 or BRCA2 gene mutations.

Women who carry the BRCA1 or BRCA2 gene mutations now have the clearest picture yet of their risk of developing breast and ovarian cancer.

An Australian study led by the University of Melbourne, Peter MacCallum Cancer Centre and Cancer Council Victoria tracked almost 10,000 women with these mutations for up to 20 years.

What they found has given clinicians "enormous power" to assess a carrier's risk of developing these cancers.

It will also provide carriers with greater confidence in the decisions they make about prevention strategies including surgery, says oncologist Professor Kelly-Anne Phillips, founder of the Peter Mac Breast and Ovarian Cancer Risk Management Clinic.

"This is the largest and most scientifically rigorous study to date without question. We used data from a large number of studies running internationally in over 18 around the world," Prof Phillips told AAP.

The study published in JAMA: The Journal of the American Medical Association found women with BRCA1 mutations have on average a 72 per cent risk of developing breast cancer by age 80.

For BRCA2 mutation carriers, the risk of breast cancer is 69 per cent.

The average lifetime risk of ovarian cancer is 44 per cent for BRCA1 and 17 per cent for BRCA2.

It was also found that the location of where the abnormality is found in the gene and family history influenced the risk.

"So women who had these gene mutations who had more people in their family who'd been affected with breast and/or ovarian cancer had a higher likelihood of developing these cancers than women with the same mutation who had less of a family history," said Prof Phillips.

Professor Phillips says the findings also have significant implications for older women with these mutations.

The study found cancer risk increased rapidly at a young age and peaks in the 30s for BRCA 1 mutation carriers and in the 40s for BRCA 2 mutation carriers.

However it remained as the woman aged, overturning previous thinking that the risk reduced when a woman turned 60 without a diagnosis.

"Our study really showed that the risk reaches its highest level in a woman's 30s and remains well into their 80s," said Prof Phillips.

Further analysis of the data will examine whether modifiable lifestyle-related factors such as timing of pregnancies, contraceptive use or alcohol and smoking can influence the risk of cancer for these women.