Lead researcher Leon Flicker of the University of Western Australia said the finding calls into question current body mass index guidelines for older adults.
The researchers looked at data taken over a decade among more than 9,200 Australian men and women, ages 70-75 at the beginning of the study, who were assessed for their health and lifestyle as part of a study into healthy aging.
Australia is ranked the third-most obese country behind the United States and the United Kingdom.
Obesity and overweight are most commonly defined according to body mass index, which is calculated by dividing body weight by the square of height.
The study began in 1996 and recruited 4,677 men and 4,563 women. The participants were tracked for 10 years or until their death.
The research uncovered the mortality risk was lowest for participants with a BMI classified as overweight, with the risk of death reduced by 13 percent compared with normal weight participants.
The benefits were only seen in the overweight category not in those people who are obese, the researchers said.
The findings are published in the Journal of The American Geriatrics Society.