The 16-year-old accessed 90 gigabytes worth of files, breaking into the system many times over the course of a year from his suburban home in Melbourne, reports The Age newspaper.
It says he stored the documents in a folder called 'hacky hack hack'.
Apple insists that no customer data was compromised.
But The Age reports that the boy had accessed customer accounts.
In a statement to the BBC, Apple said: "We vigilantly protect our networks and have dedicated teams of information security professionals that work to detect and respond to threats.
"In this case, our teams discovered the unauthorised access, contained it, and reported the incident to law enforcement.
"We regard the data security of our users as one of our greatest responsibilities and want to assure our customers that at no point during this incident was their personal data compromised."
Well-known hacker According to statements made in court, the smartphone giant contacted the FBI when it became aware of the intrusion, and the matter was referred to the Australian Federal Police (AFP).
An AFP raid on the boy's home revealed two laptops with serial numbers matching those of devices which had accessed the system. Police also seized a mobile phone and a hard drive.
According to The Age, the teen had boasted about his activities in WhatsApp messages. It reports that he had hacked into the firm because he was a huge fan and dreamed of working there.
His defence lawyer said that he had become very well-known in the international hacking community.
The boy's name has not been made public for legal reasons. He is due to be sentenced on 20 September.