The UN's special rapporteur on extreme poverty and human rights is reported to have written to the government following the decision to move about 84,000 parents to the unemployment Newstart allowance when their youngest child turns eight.
More than 60,000 single parents now receive between $60 to $100 a week less under the change.
"There was no court for us to go in Australia," ACOSS chief executive Cassandra Goldie told ABC Radio on Monday.
Beth Goldblatt from the Australian Human Rights Centre said the government should seriously consider the concerns of the UN's special rapporteur.
Parliament's human rights committee has recommended the government delay the change.
The UN monitor is reported to have been waiting for a reply from the government for more than four months.
Comment is being sought from the government.