Hollywood star Sylvester Stallone has pleaded guilty to bringing 48 vials of banned muscle-building hormones into Australia.

In Sydney earlier today his lawyer, Phillip Boulten, handed a letter to the court in which the actor admitted two charges of importing a prohibited human growth hormone.

"I made a terrible mistake. Not because I was attempting to deceive anyone but I was simply ignorant of your official rules," Stallone wrote.

The 60-year-old star of the 'Rocky' and 'Rambo' films was not in the court for the hearing.

The vials were found during a customs search of his luggage at the start of a three-day visit to Sydney in February.

The court was also told that Stallone later threw four vials of the male hormone testosterone from his hotel room when customs officials arrived to search it.

The actor faces a maximum penalty of AUS$22,000 on each of the two charges. He will be sentenced on Monday next week.