Timothy David Deacon, 26, faced the ACT Magistrates Court on Friday on 17 counts of misusing and attempting to misuse a Commonwealth credit card.

Court documents said Deacon was issued with a travel card in 2011 for his work with the Defence Department.

Defence employees are authorised to use the card to pay for work-related travel, including transport, accommodation, and meals.

But Deacon instead used the account as ready cash to punt on the poker machines.

He used the card on 16 occasions over a four-week period between August and September last year, withdrawing about $25,000 in cash.

The withdrawal fees and charges took the total loss to the Commonwealth to $25,496.75.

Deacon attempted to make a further four cash withdrawals on September 12 but failed as the card had been cancelled.

The defendant admitted the fraud to his supervisor during a phone call later that day.

Deacon then wrote a letter to a departmental chief saying: "I recently accessed funds from my [card] to feed a gambling addiction which I thought I had beaten. I write to apologise to you for this dreadful lapse in judgment that I have inflicted on my colleagues and Defence."

In October, he voluntarily took part in a recorded interview where he admitted the thefts.

Deacon said he had a gambling addiction since 2009 and had undergone professional counselling to treat the condition before joining Defence.

Magistrate Karen Fryar ordered the defendant participate in a pre-sentence report.

The Commonwealth has sought a reparation order to have the money repaid. Deacon will be sentenced in December.