Nearly 750,000 welfare recipients were overpaid by Centrelink in the second half of last year, creating what welfare advocates say is a "bonanza" for debt collectors.

Figures indicate that one in 10 of all recipients, including students on Austudy and Abstudy, youth allowance and various parenting payments received more than they were entitled to between July and December 2014.

The biggest class of over-payment was family tax benefit, followed by Newstart and the single parent payment. Credit:Jim RIce

The biggest classes of overpayments were family tax benefit, followed by Newstart allowance and the single-parent payment.

More than 350,000 families were overpaid family tax benefit. The giant Department of Human Services said debts from overpayments had accrued as a result of families either underestimating their annual income, failing to lodge tax returns or failing to notify the department of changed circumstances.