According to Australian press wires, Harrison was charged last August with creating the trojan, since when the authorities have been developing the case against him.
The Sydney Morning Herald reports that Harrison pleaded guilty in the South Australian District Court on Monday of this week to seven charges – including four counts of modifying computer data to cause harm and possessing data to commit a serious computer offence.
The paper says that the court had previously heard that "Harrison had the capability to strike a further 74 000 computers around the world and bring down international websites."
Plans now call for Harrison to be sentenced in late September.
The case is an interesting one on several levels, Infosecurity notes, as the South Australian Police Electronic Crime Section arrested Harrison in August of last year with the assistance of the Australian Federal Police High Tech Crime Operations branch.
At the time, police alleged that he had compromised more than 3000 computers throughout the world by infecting them with software designed to capture banking credentials and credit card information.
In a statement released by police on the day of his arrest, police said he had developed capabilities to launch attacks on up to 74 000 computers worldwide.
"The arrest has resulted in the acquisition of intelligence which can be utilised to identify other offenders", a police spokesperson said at the time.