IT WAS the first time that Patricia Mary Butler had ventured back to the streets since she was sexually assaulted by a client two weeks earlier.

But in her fragile emotional state, she could not have landed a worse client than the one who pulled up to the corner of Bourke and William streets in the early hours of July 31, 2010. And with his particular fetish, he could not have landed a worse girl.

Brendan Potter was into child sex, and fantasised about it while Butler performed sex acts in Rushcutters Bay Park, and later at her Potts Point apartment.

He had no idea that Butler's stepfather had repeatedly sexually assaulted her as a girl, that he had threatened to rape her little sister if she told anybody, and that the recent attack she endured had powerfully reminded her of those childhood molestations.

So, when Mr Potter conjured a detailed fantasy about performing sex acts on her younger sister, she snapped.