In its 2012/13 federal budget, the Labor government made changes to parenting payment eligibility which will mean single parents will be moved to the Newstart Allowance when their youngest child turns eight.
With the legislation about to be introduced to parliament, the Australian Council of Social Service and 12 other welfare organisations and charities have penned a joint letter to all MPs and senators urging them to vote down the measure.
The letter, obtained by AAP, warns scores of single parent families will be $30 to $40 worse off each week if the measure passes parliament even when the cuts are offset by other payments such as the schoolkids bonus.
"There is no evidence to show that moving sole parents and their children onto lower payments will increase their job prospects, and in any event this is the harshest approach to welfare reform," the letter says.
The letter says sole parent families with a school-age child living on parenting payments are already living on less than $500 a week.
"Any payment reductions will only push them deeper into poverty," the welfare groups said.
Ninety per cent of sole parent families are headed by mothers, the letter says.
The welfare groups also expressed their disappointment the government had ignored calls to lift Newstart payments by $50 a week to stop job seekers from falling below the poverty line.
St Vincent de Paul, Salvation Army, Uniting Care and Mission Australia are among the organisations who have signed the letter.
Workplace Minister Bill Shorten is expected to introduce the Social Security Legislation Amendment (Fair Incentives to Work) Bill 2012 to the lower house today.