According to study leader, Nicole Au, a research fellow at the Centre for Health Economics at Monash University, the weight gain was likely linked to the women living less healthy lifestyles, the Sydney Morning Herald reported.
"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 said.
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.
About 65 percent who worked long hours drank at risky levels, compared with 42 percent of the women who were not in the labour force and 53 percent who were unemployed.
"These statistics ... provide some clues as to how employment patterns may affect lifestyle choices, and subsequently, body weight," Dr Au wrote.
Fifty-five percent of the women gained weight over two years and 31 percent lost weight.
"On average the women gained 1.5 per cent of their initial weight but we were seeing quite extreme amounts of weight gain as well," she said.
Dr Au said the effect of long work hours was particularly noticeable among the women who gained the most weight, with some gaining nearly 12 percent of their body weight - the equivalent of about eight kilograms for a woman weighing 69 kilograms.
The finding could have serious health implications because larger changes in weight were likely to have more serious health impacts, Dr 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.
The study is published in the International Journal of Obesity.