An Australian study has found being physically active won't protect you against developing type 2 diabetes if you are already overweight or obese.

Maintaining a healthy weight is the single most effective way to reduce your risk of type 2 diabetes, no matter how much you hit the gym, new research has found.

A large Australian study of more than 30,000 people has found being physically active won't protect you from developing the disease if you are already overweight or obese.

According to the research, the people who were obese - even if they were physically active and spent little time sitting - had five times the risk of developing type 2 diabetes compared to people of normal weight, even those who had lower levels of physical activity and who sat more.

People who were overweight had twice the risk as people who were of normal weight and less active.

The Sax Institute's 45 and Up Study, presented in Sydney on Tuesday, debunks the myth that being 'fat but fit' will reduce your risk of type 2 diabetes.

Lead researcher Thanh-Binh Nguyen from the University of Sydney says their research suggests being physically active is not as important as maintaining a healthy weight when it comes to preventing diabetes and highlights the importance of a healthy diet.

"Once you are overweight being physical active doesn't help you that much in terms of preventing type 2 diabetes. It helps you if you can manage to reduce your weight, so it's important to continue to be physically active and to adopt a healthy diet."

One of the most important aspects of diabetes management is to maintain a healthy body weight, advises Diabetes Australia.

Being overweight not only increases your risk of heart disease, stroke and some cancers, it also makes your diabetes harder to manage.

Previous research has found that if you have pre-diabetes (impaired fasting glucose or impaired glucose tolerance), losing 5 to 10 per cent of your current body weight can prevent type 2 diabetes in up to nearly 6 out of 10 people.

This equates to losing five to 10 kilos for a person who weights 100kg.

Diabetes Australia says small changes in diet, such as reducing portion sizes and swapping to low-fat dairy products, can help people to achieve a healthy body weight and manage diabetes.