Taxpayers are now paying for seven lawyers to help the federal government keep details secret of a $6000 child support dispute that has now cost taxpayers more than $565,000 in legal fees.

But despite its expensive taxpayer-funded legal team, the giant Department of Human Services could have its case thrown out because of a failure to properly fill in a simple form.

The Administrative Appeals Tribunal has been told that high-end law firm Clayton Utz, hired by the department, failed to properly complete the tribunal's application form, designed for ease of use by people with no legal training.

The legal firm is battling to overturn an order from the government information watchdog to the department to hand over documents to a father who has spent four years disputing a Child Support Agency orders that he pay $6000 to his ex-wife.

Fairfax Media revealed last year that Child Support Agency bosses had spent more than $500,000 despite knowing, since August 2011, that their public servants broke the law in the man's case and were on shaky legal ground from the beginning of the dispute.

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That figure is now expected to grow even further with DHS still fighting the father, a determined litigant known only as DT because of the strict privacy provision surrounding family law, in both the Family Court and the AAT.

The department's legal teams, all paid for by taxpayers, include a barrister, a special counsel from Clayton Utz, a principal solicitor from the firm, a senior lawyer from the Australian Government Solicitor's office and a more junior colleague and two instructing solicitors from Human Services.

In addition to the Administrative Appeals action, the department is also battling in the Family Court to block the father's access to key departmental documents in the case, which has been described by independent Senator Nick Xenophon as a  "scandalous waste of taxpayers' money … to protect the butt of the department".

In its application to the AAT, the department's lawyers said the decision of the Australian Privacy Commissioner ordering the department to hand the documents over to the father was wrong, but failed to articulate why it was wrong.

DT argued in a hearing in Sydney on Monday that the failure to make out its application properly meant that the case should be thrown out.

In its defence Clayton Utz submitted that under the relevant legislation, it does not have to answer the section of the application form asking for the reasons for the administrative challenge.

Deputy President James Constance reserved his decision while a judgement in the Family Court case as reserved in February 2014.

A Human Service spokeswoman declined on Tuesday to discuss her department's conduct of the case.

"As this matter is still subject to ongoing litigation action at the Administrative Appeals Tribunal, it is not appropriate for us to comment on any aspects related to this case," she said.