One such network called 'Human Milk 4 Human Babies' connects parents in need of breast milk with those willing to provide it free of charge.
Users in Australia have responded to requests for milk donations for babies as young as three weeks.
In one case, users also donated to a man with cancer, whose wife appealed for human milk after their doctor recommended it to boost his immune system.
Experts, however, warn of the risks involved in sharing milk, which has the potential to be a carrier for bacteria and viruses, including HIV.
Director of Mothers' Milk Bank, Marea Ryan, said peer-to-peer milk-sharing was potentially dangerous, and urged mothers to use screened and registered milk bank services.
"The risks are of those viruses passing through to the baby (and) if they haven't had good hygiene at the point of collecting and expressing the milk there's potential there for bacteria to be in the milk," News.Com.Au quoted Ryan as saying.
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Tweed-based not-for-profit Mothers' Milk Bank provides screened and pasteurised breast milk at an administration cost of 80 dollars for 1.2 litres and rarely has problems with supply, she said.
Ryan noted that it was understandable that mothers turned to peer-to-peer milk-sharing if they couldn't cover the cost, and called on the federal and state governments to fund milk banks across the Australia.