Daniel Houghton, 25, tried to sell the electronic files to the Netherlands for $A3.43m, but Dutch agents filmed him as he displayed the material and passed on the evidence to MI6, the Royal Courts of Justice on London heard.
Houghton denied a charge of stealing the electronic files, which was accepted by the court.
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Judge David Bean said a prison term was "inevitable" and gave the prosecution permission to obtain a psychiatric assessment of Houghton ahead of sentencing on September 3.
The court heard that when Houghton offered the information to the Dutch agents in March, the price was negotiated down to $A1.55m.
But immediately after Houghton handed over the files he was arrested as he left with the cash in a suitcase.
In searches of his flat, police found a computer memory stick containing 7,000 files and paperwork marked "top secret".
