Besides this, the NAO has also claimed it found irregular payments and receipts, outstanding arrears and issues with financial reporting.
The Department for Work and Pensions (DWP) took control of the account responsible for recording the amounts of child maintenance from non-resident parents and parents with care in July 2012 after the Child Maintenance and Enforcement Commission was abolished.
According to the NAO, since DWP became responsible there have been issues with the main IT systems that support the schemes, such as a lacking the functionality to report arrears for the account in full.
Because of this, DWP has now put a separate process in place that scans the underlying systems and produces reports. It hopes that the recently introduced new process, combined with a new IT system, will decrease administrative complexity and reliance on manual processes.
In 2012/13, the department says it received £827.3m in child maintenance. However, the NAO says it has discovered overpayments of £7.6m and underpayments of £6.1m in that year.
The office also estimates that there is £3.8bn in outstanding arrears from non-resident parents dating back to the start of the schemes in 1993.
“Current legislation does not allow for the writing-off of outstanding arrears. These figures do not give a true and fair view because of the level of error in the underlying case data,” said the NAO.
“The best available estimates of the cumulative errors indicate that the reported arrears at 31st March 2012 contained overstatements of £210m and understatements of £319m,” it added.