"There are concerns the legislation will have significant and detrimental impacts on the human rights of over 100,000 Australians," the letter from the UN reads.
The United Nations Rapporteur on extreme poverty and human rights says Australia may be in breach of its international obligations, by putting 84,000 single parents on the Newstart allowance.
The allowance sees parents pocket up to $100 a week. Parents on The Single Mum forum say they are already struggling to make ends meet, and hope the global attention brought on by the UN correspondence will force the government to change the policy.
"I'm relieved that an organisation with as great a standing as the UN has actually weighed in on this," says single mum Kristie Rue.
"Potentially, the government might actually listen to them, because they're not listening to us."
"The UN bringing this out and asking for a response from the government is making headline news and potentially making it an international issue," Ms Rue says.
The Greens want a $50 a week increase to the Newstart allowance.
Labor says the Newstart allowance simply brings consistency to single parenting payments, with some people receiving more than others until January 1st this year.
The measure will save the government $700 million in the budget.
The UN Rapporteur says because 90 per cent of single parents are women, the legislative changes unfairly targets women.
The UN also warned the government that the changes may cause it to be in breach of its international obligations.