Researchers said on Thursday diet is more important than exercise for people wanting to lose weight.

The study revealed that it was time to bust the myth of physical inactivity and obesity: "you cannot outrun a bad diet."

Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO) research director Professor Manny Noakes said sugar and carbs alone were responsible for obesity.

"Australians eat over 30 percent of excess kilojoules from indulgence foods," she said.

"Some may be high in sugar and carbohydrate, but many may also be high in fat and salt.

"Eating for good health and well-being will not be achieved by focusing on avoidance of one nutrient."

Exercise has many benefits, but particularly for weight loss maintenance rather than weight loss, she added.

The report, which was published in the British Journal of Sports Medicine, said regular exercise is key to staving off serious disease, such as diabetes, heart disease and dementia.

But they say calorie laden diets now generate more ill health than physical inactivity, alcohol and smoking combined.