Children are 20 percent more likely to become overweight if their mothers go from not working to working more than 20 hours a week, according to research from UNSW.

The research is the first in Australia to make the link.

The work, by a fourth year honours student in the Faculty of Business, Anna Zhu, analyses data about 5,000 children between the ages of four and five and their families. The data was compiled for the Federal government's Longitudinal Survey of Australian Children.

Anna's research finds that while there is a substantial and damaging effect once a mother starts working more than 20 hours a week, there is no effect on a child's weight if their mother works less than that.

Her work found there is no correlation between the hours that fathers work and the weight of their child or children.

"Higher household incomes reduce the likelihood of a child being overweight and lessen the damaging impacts associated with a mother moving from non-working to full-time employment," said Ms Zhu, in the report.

"It is the actions adopted by the full-time mother in response to time constraints and the struggle to fill her dual role as child care giver and economic provider that is partly the cause of the higher rates of overweight children in Australia over the last few decades," the paper argues.

The Australasian Society for the Study of Obesity (2004) found that childhood obesity in Australia is increasing at 1 per cent per year and that half of all Australian children will be overweight by 2025.

The work was supervised by UNSW Professor of Economics, Denzil Fiebig.