Dr Lisa Amir, from Mother and Child Research at La Trobe University, and her co-authors investigated whether the relationship between socioeconomic status and breastfeeding initiation and duration had changed between 1995 and 2004.
While the overall duration of breastfeeding remained fairly constant, the gap between the most and least disadvantaged families widened considerably over this period.
In 1995, 37.7 per cent of women in the lowest socioeconomic group were breastfeeding their babies at 6 months, compared with 53.1 per cent of women in the highest socioeconomic group. In 2004/05, the proportions in these two groups were 37.1 per cent and 66 per cent, respectively.
Dr Amir said infants in higher socioeconomic groups were more likely to be breastfed, as their mothers often adopted healthier behaviour such as exercising, eating healthy diets and quitting smoking.
Women from lower socioeconomic backgrounds were less likely to breastfeed for various reasons, including a lack of family support, less ability to seek help with breastfeeding problems, less flexibility with working arrangements, and concerns about breastfeeding in public.
"Moreover, women [with lower socioeconomic status] are more likely to interact socially with women who are less inclined to breastfeed, such as those who are younger, less educated, overweight/obese or smokers," she said.
Dr Amir warned that, as formula-fed infants were more likely to become ill and be admitted to hospital, her group's research indicated increasing health inequalities in Australian children.
"New mothers need support from their families, communities and workplaces in order to breastfeed.
"They need Baby Friendly accredited maternity hospitals, increased breastfeeding help in the community and paid maternity leave - not simply another government campaign extolling the virtues of breastfeeding," she said.
The researchers called for policymakers to focus breastfeeding support and promotion on groups with low initiation rates and to trial peer support programs that have been effective in other countries - for example, teenage mothers supporting other teenage mothers.
She said there should also be a public education campaign to promote breastfeeding in public in an acceptable way to groups that are currently uncomfortable with this issue.