The weight gain was likely linked to the women living less healthy lifestyles, says the study leader, Nicole Au, a research fellow at the Centre for Health Economics at Monash University.
"Working women are faced with so many different time pressures, which leaves them with less time to engage in physical activity and less time to prepare healthy meals," she told the Sydney Morning Herald.
Women who worked long hours, defined as between 41 and 48 hours, or very long hours, meaning more than 49 hours a week, were also more likely to smoke, drink at risky levels and not exercise.
Au said it was important for women to be aware that if they were working very long hours that their health could be at risk.