A study of Melbourne cancer patients has found a positive association between regular soft drink consumption and an increased risk of developing cancer.

People who regularly consume at least one soft drink a day, no matter the size of their waist, could be at increased risk of cancer, according to a new study.

Researchers at Cancer Council Victoria and University of Melbourne analysed more than 3000 cases of 11 obesity-related cancers including breast, liver and prostate reported between 1990-1994 and 2003-2007 through The Melbourne Collaborative Cohort Study.

Published on Thursday, the study found a positive association between soft drink consumption and cancer risk independent of obesity after statistically adjusting for waist circumference.

The authors say the results justify the need to minimise the intake of sugar-sweetened soft drinks.

"Initially our hypothesis was that drinking soft drinks would cause obesity which would then cause an association with obesity-related cancers but we found that there was more beyond the affect of obesity," said lead researcher, Associate Professor Allison Hodge of Cancer Council Victoria's Cancer Epidemiology and Intelligence Division.

"These particular cancers are commonly associated with obesity, however, our research found this risk existed for all participants, no matter their size," said Professor Hodge.

According to the research, the more sugary soft drinks participants drank the higher their risk of cancer.

However, this was not the case with those who drank diet soft drinks, suggesting sugar could be the key, says Professor Hodge.

Professor Hodge says the "surprising" findings warrant further research.

"We definitely would like to see these findings confirmed in other studies and then also to understand something about the mechanism and whether sugar could be a driver of this association," said Professor Hodge.

Cancer Council Victoria CEO Todd Harper says this new research provides another reason for Australians to avoid soft drinks and make the switch to water.

"Sugary drinks, including soft drinks, are already known to be a cause of obesity, which greatly increases the risk of 13 types of cancer," said Mr Harper.

"And cancer is just one of many chronic health conditions associated with sugary drink consumption- including increased risk of type 2 diabetes, heart disease, and tooth decay."