The stalker managed to hack into the girl's Facebook account / AAP Source: AAP

But a month-long investigation by the Australian Federal Police found the girl was in fact being stalked by another 12-year-old girl, The Sydney Morning Herald reported.

The Sydney girl and her mother had been subjected to a shocking two-week ordeal in which they were sent pornographic images and had abusive messages posted on their Facebook pages.

The girl’s mother had reported the matter to police and the social networking site in a desperate bid to find the culprit and stop the abuse.

But she could not get Facebook to intervene to stop the harassment.

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The case was dropped, however, after the identity of the stalker was revealed.

The abuse began last month after someone hacked into the girl's Facebook profile and used the account to try to get her friends to divulge information about her.

The person also knew where the family lived and used the girl’s profile to post threatening messages on her mother’s profile.

One message read: "f***ing give me your daughters they love me and i love them, you know you cant keep me away from them forever! i am going to be with them they are the mother of my children."

"i will find ur girls," another post said.

It is not known how the girl’s account was hacked.

While Facebook does not have an office in Australia, its website usage policy states that users should be 13 years or older.