MEN are using a new law to win back hundreds of thousands of dollars paid in child support for children they later discovered were not theirs.

New figures obtained by The Australian reveal that since January 2007, 48 men have won back a total of $434,378.64 paid through the Child Support Agency.

The men used DNA testing to prove they were not the biological fathers of children they had been supporting. One man got back more than $70,000 by proving children were not his.

But among the 48 orders, the sums to be repaid by mothers ranged down to less than $20.

Section 143 of the Child Support (Assessment) Act, requires the Family Court to consider issuing orders for repayment where paternity is successfully challenged and child support has been paid.

Child Support Agency deputy secretary Philippa Godwin said the agency had received no complaints about the process, but women's groups are outraged by the numbers.

"Where parentage has been satisfied for child support purposes, but a parent believes they are not a parent of a child, they can apply to a court for a declaration that a child support assessment cannot be made against them for that child," Ms Godwin said.

"As part of determining this application, the court can order a DNA test to determine paternity of the child. The results of the test will be considered by the court in deciding whether to make the declaration."

Hundreds of tests have proved that an aggrieved man is, in fact, the biological father of the child in question.

Sole Parents Union president Kathleen Swinbourne said the new laws were hurting children at the centre of DNA disputes.

"What does this say to children about being wanted, being loved, being parented? What does this do to children whose fathers turn around and say, `I'm not your daddy anymore, I don't want you'," she said.

"The effect of this on children must be devastating. And that's to say nothing about the financial circumstances mothers find themselves in in trying to continue to support their children while trying to pay back child support."

Ms Swinbourne said she did not accept the argument that men should be entitled to change their role in children's lives based on biology.

"If you've raised these children and parented them, you can't turn around later and change your mind. It doesn't matter what the DNA says."

Men's Rights Agency director Sue Price said it was about time that men who had been duped by former partners were able to remedy the situation.

"I think it's a good thing that children are able to know the biological father rather than their pretend father," she said. "I think it's essential that they should be able to have the money repaid to them. Why should they have to pay for another man's child?"